diff --git a/lib/appenders/gelf.js b/lib/appenders/gelf.js
index 4599856..0a731db 100644
--- a/lib/appenders/gelf.js
+++ b/lib/appenders/gelf.js
@@ -34,22 +34,61 @@ levelMapping[levels.FATAL] = LOG_CRIT;
  * @param hostname - hostname of the current host (default:os hostname)
  * @param facility - facility to log to (default:nodejs-server)
  */
-function gelfAppender (layout, host, port, hostname, facility) {
+function gelfAppender (layout, host, port, hostname, facility, customFields) {
   
   host = host || 'localhost';
   port = port || 12201;
   hostname = hostname || require('os').hostname();
   facility = facility || 'nodejs-server';
   layout = layout || layouts.messagePassThroughLayout;
+
+  var defaultCustomFields = customFields || {};
   
   var client = dgram.createSocket("udp4");
   
   process.on('exit', function() {
     if (client) client.close();
   });
-  
+
+  /**
+   * Add custom fields (start with underscore ) 
+   * - if the first object passed to the logger contains 'GELF' field, add copy the underscore fields to the message
+   * @param loggingEvent
+   * @param msg
+   */
+  function addCustomFields(loggingEvent, msg){
+
+      /* append defaultCustomFields firsts */
+      var keys = Object.keys(defaultCustomFields);
+      for (var i in keys){
+          var key = keys[i];
+          // skip _id field for graylog2, skip keys not starts with UNDERSCORE
+          if (!key.match(/^_/) || key === "_id") continue; 
+          msg[key] = defaultCustomFields[key];
+      }
+
+
+      /* append custom fields per message */
+      var data = loggingEvent.data;
+      if (!Array.isArray(data) || data.length === 0) return;
+      var firstData = data[0];
+
+      if (!firstData['GELF']) return; // identify with GELF field defined
+      var keys = Object.keys(firstData);
+      for (var i in keys){
+          var key = keys[i];
+          // skip _id field for graylog2, skip keys not starts with UNDERSCORE
+          if (!key.match(/^_/) || key === "_id") continue; 
+          msg[key] = firstData[key];
+      }
+
+      /* the custom field object should be removed, so it will not be looged by the later appenders */
+      loggingEvent.data.shift(); 
+  }
+ 
   function preparePacket(loggingEvent) {
     var msg = {};
+    addCustomFields(loggingEvent, msg);
     msg.full_message = layout(loggingEvent);
     msg.short_message = msg.full_message;
     
@@ -88,7 +127,7 @@ function configure(config) {
   if (config.layout) {
     layout = layouts.layout(config.layout.type, config.layout);
   }
-  return gelfAppender(layout, config.host, config.port, config.hostname, config.facility);
+  return gelfAppender(layout, config.host, config.port, config.hostname, config.facility, config.customFields);
 }
 
 exports.appender = gelfAppender;
diff --git a/test/gelfAppender-test.js b/test/gelfAppender-test.js
index 1f45f33..7a0541b 100644
--- a/test/gelfAppender-test.js
+++ b/test/gelfAppender-test.js
@@ -135,5 +135,51 @@ vows.describe('log4js gelfAppender').addBatch({
         assert.equal(message.facility, 'nonsense');
       }
     }
+  },
+  'with custom fields options': {
+    topic: function() {
+      var setup = setupLogging({
+        host: 'somewhere',
+        port: 12345,
+        hostname: 'cheese',
+        facility: 'nonsense',
+        customFields: {
+            _every1: 'Hello every one',
+            _every2: 'Hello every two'
+        }
+      });
+      var myFields = {
+          GELF: true,
+          _every2: 'Overwritten!',
+          _myField: 'This is my field!'
+      };
+      setup.logger.debug(myFields, "Just testing.");
+      return setup;
+    },
+    'the dgram packet': {
+      topic: function(setup) {
+        return setup.dgram;
+      },
+      'should pick up the options': function(dgram) {
+        assert.equal(dgram.socket.host, 'somewhere');
+        assert.equal(dgram.socket.port, 12345);
+      }
+    },
+    'the uncompressed packet': {
+      topic: function(setup) {
+        var message = JSON.parse(setup.compress.uncompressed);
+        return message;
+      },
+      'should pick up the options': function(message) {
+        assert.equal(message.host, 'cheese');
+        assert.equal(message.facility, 'nonsense');
+        assert.equal(message._every1, 'Hello every one'); // the default value
+        assert.equal(message._every2, 'Overwritten!'); // the overwritten value
+        assert.equal(message._myField, 'This is my field!'); // the value for this message only
+        assert.equal(message.short_message, 'Just testing.'); // skip the field object 
+        assert.equal(message.full_message, 'Just testing.'); // should be as same as short_message 
+      }
+    }
   }
+
 }).export(module);