diff --git a/config/default.js b/config/default.js
index a6fb1dc..f9559ae 100644
--- a/config/default.js
+++ b/config/default.js
@@ -3,7 +3,7 @@
 module.exports = {
 
   log: {
-    level: 'silly'
+    level: process.env.LOG_LEVEL || 'silly'
   },
 
   server: {
diff --git a/index.js b/index.js
index fe6da9c..33fa395 100644
--- a/index.js
+++ b/index.js
@@ -20,7 +20,7 @@
 const cluster = require('cluster');
 const numCPUs = require('os').cpus().length;
 const config = require('config');
-const log = require('npmlog');
+const log = require('winston');
 
 log.level = config.log.level;
 
diff --git a/package.json b/package.json
index 9e27f6a..0deb600 100644
--- a/package.json
+++ b/package.json
@@ -29,8 +29,8 @@
     "mongodb": "^2.2.31",
     "nodemailer": "^2.4.2",
     "nodemailer-openpgp": "^1.0.2",
-    "npmlog": "^4.0.2",
-    "openpgp": "^2.3.0"
+    "openpgp": "^2.3.0",
+    "winston": "^2.3.1"
   },
   "devDependencies": {
     "chai": "^4.1.1",
diff --git a/src/app/middleware.js b/src/app/middleware.js
index 533ca1b..d56fc74 100644
--- a/src/app/middleware.js
+++ b/src/app/middleware.js
@@ -17,7 +17,7 @@
 
 'use strict';
 
-const log = require('npmlog');
+const log = require('winston');
 const config = require('config');
 const koaBody = require('koa-body');
 const util = require('../service/util');
@@ -58,7 +58,7 @@ exports.parseBody = () => koaBody({
 
 exports.logUnknownError = function(error, ctx) {
   if (error.status) {
-    log.verbose('middleware', 'Request faild: %s, %s', error.status, error.message);
+    log.verbose('middleware', `Request failed: ${error.status} ${error.message}`);
   } else {
     log.error('middleware', 'Unknown error', error, ctx);
   }
diff --git a/src/dao/mongo.js b/src/dao/mongo.js
index 12f5170..8107a2d 100644
--- a/src/dao/mongo.js
+++ b/src/dao/mongo.js
@@ -17,7 +17,7 @@
 
 'use strict';
 
-const log = require('npmlog');
+const log = require('winston');
 const MongoClient = require('mongodb').MongoClient;
 
 /**
diff --git a/src/email/email.js b/src/email/email.js
index dcaf996..86ee2a8 100644
--- a/src/email/email.js
+++ b/src/email/email.js
@@ -17,7 +17,7 @@
 
 'use strict';
 
-const log = require('npmlog');
+const log = require('winston');
 const util = require('../service/util');
 const nodemailer = require('nodemailer');
 const openpgpEncrypt = require('nodemailer-openpgp').openpgpEncrypt;
diff --git a/src/index.js b/src/index.js
index a5986bd..05c247d 100644
--- a/src/index.js
+++ b/src/index.js
@@ -17,7 +17,7 @@
 
 'use strict';
 
-const log = require('npmlog');
+const log = require('winston');
 const config = require('config');
 const init = require('./app');
 
diff --git a/src/service/pgp.js b/src/service/pgp.js
index f7c049a..95c8a2f 100644
--- a/src/service/pgp.js
+++ b/src/service/pgp.js
@@ -17,7 +17,7 @@
 
 'use strict';
 
-const log = require('npmlog');
+const log = require('winston');
 const util = require('./util');
 const openpgp = require('openpgp');
 const addressparser = require('addressparser');
diff --git a/test/integration/app-test.js b/test/integration/app-test.js
index bd8a09e..2edf598 100644
--- a/test/integration/app-test.js
+++ b/test/integration/app-test.js
@@ -5,7 +5,7 @@ const Mongo = require('../../src/dao/mongo');
 const nodemailer = require('nodemailer');
 const config = require('config');
 const fs = require('fs');
-const log = require('npmlog');
+const log = require('winston');
 
 describe('Koa App (HTTP Server) Integration Tests', function() {
   this.timeout(20000);
diff --git a/test/integration/mongo-test.js b/test/integration/mongo-test.js
index ef20411..1e88d08 100644
--- a/test/integration/mongo-test.js
+++ b/test/integration/mongo-test.js
@@ -1,6 +1,6 @@
 'use strict';
 
-const log = require('npmlog');
+const log = require('winston');
 const config = require('config');
 const Mongo = require('../../src/dao/mongo');
 
diff --git a/test/integration/public-key-test.js b/test/integration/public-key-test.js
index cf5f8a0..2d88ef4 100644
--- a/test/integration/public-key-test.js
+++ b/test/integration/public-key-test.js
@@ -1,6 +1,6 @@
 'use strict';
 
-const log = require('npmlog');
+const log = require('winston');
 const config = require('config');
 const nodemailer = require('nodemailer');
 const Email = require('../../src/email/email');
diff --git a/test/unit/email-test.js b/test/unit/email-test.js
index 68375a9..83a93eb 100644
--- a/test/unit/email-test.js
+++ b/test/unit/email-test.js
@@ -1,6 +1,6 @@
 'use strict';
 
-const log = require('npmlog');
+const log = require('winston');
 const Email = require('../../src/email/email');
 const nodemailer = require('nodemailer');
 
diff --git a/test/unit/pgp-test.js b/test/unit/pgp-test.js
index 338ef74..43e386f 100644
--- a/test/unit/pgp-test.js
+++ b/test/unit/pgp-test.js
@@ -1,7 +1,7 @@
 'use strict';
 
 const fs = require('fs');
-const log = require('npmlog');
+const log = require('winston');
 const openpgp = require('openpgp');
 const PGP = require('../../src/service/pgp');