Despite remarkable progress, weakly supervised segmentation methods are still inferior to their fully supervised counterparts. We obverse that the performance gap mainly comes from the inability of producing dense and integral pixel-level object localization for training images only with image-level labels. In this work, we revisit the dilated convolution proposed in  and shed light on how it enables the classification network to generate dense object localization. By substantially enlarging the receptive fields of convolutional kernels with different dilation rates, the classification network can localize the object regions even when they are not so discriminative for classification and finally produce reliable object regions for benefiting both weakly- and semi- supervised semantic segmentation. Despite the apparent simplicity of dilated convolution, we are able to obtain superior performance for semantic segmentation tasks. In particular, it achieves 60.8\% and 67.6\% mean Intersection-over-Union (mIoU) on Pascal VOC 2012 test set in weakly- (only image-level labels are available) and semi- (1,464 segmentation masks are available) settings, which are the new state-of-the-arts.