diff --git a/mmdet/datasets/builder.py b/mmdet/datasets/builder.py
index a8b0e29c5dac8cccf7a8ea125dad20354ddd141e..1d101ff93e17a24a1193285ee999dc142a56aa11 100644
--- a/mmdet/datasets/builder.py
+++ b/mmdet/datasets/builder.py
@@ -28,7 +28,9 @@ def _concat_dataset(cfg, default_args=None):
 
 
 def build_dataset(cfg, default_args=None):
-    if cfg['type'] == 'RepeatDataset':
+    if isinstance(cfg, (list, tuple)):
+        dataset = ConcatDataset([build_dataset(c, default_args) for c in cfg])
+    elif cfg['type'] == 'RepeatDataset':
         dataset = RepeatDataset(
             build_dataset(cfg['dataset'], default_args), cfg['times'])
     elif isinstance(cfg['ann_file'], (list, tuple)):