diff --git a/mmdet/apis/train.py b/mmdet/apis/train.py
index fee47d6d2a9e66a5c59e4e98b2f0c1c3522080e2..951f251a00d56fa32801861a93e1955f1d0a246e 100644
--- a/mmdet/apis/train.py
+++ b/mmdet/apis/train.py
@@ -141,6 +141,11 @@ def build_optimizer(model, optimizer_cfg):
 
 
 def _dist_train(model, dataset, cfg, validate=False):
+    if validate:
+        raise NotImplementedError('Built-in validation is not implemented '
+                                  'yet in not-distributed training. Use '
+                                  'distributed training or test.py and '
+                                  '*eval.py scripts instead.')
     # prepare data loaders
     dataset = dataset if isinstance(dataset, (list, tuple)) else [dataset]
     data_loaders = [