Recognizing micro-expressions underpins significant and critical research and significant application. We speculate that this problem requires the understanding of the subtle face movement, integration of face structures and a solution of limited training data. In this paper, we build an effective micro-expression recognition system that leverages techniques stemming from these speculations. First, we introduce an optical flow method based on the onset frame and the apex frame to encode the subtle face motion. This has already been validated by prior research. Second, to obtain discriminative representations from the rigid face structures, part-based average pooling is proposed to inject structure priors to the network. Finally, because the system suffers from small training sets, we propose to transfer domain knowledge from macro-expression recognition tasks to micro-expression recognition. Specifically, we adopt two domain adaptation techniques including adversarial training and expression magnification and reduction (EMR). Through experiment, we show that the proposed system achieves very competitive results on the 2^nd Micro-Expression Grand Challenge (MEGC).