Abstract lenging problem in visual information retrieval, which automatically generates the natural language answer from the referenced video content according to the question. However, the existing visual question answering works only focus on the static image, which may be ineffectively applied to video question answering due to the lack of modeling the temporal dynamics of video contents. In this paper, we consider the problem of open-ended video question answering from the viewpoint of spatio-temporal attentional encoderdecoder learning framework. We propose the hierarchical spatio-temporal attention network for learning the joint representation of the dynamic video contents according to the given question. We then develop the spatio-temporal attentional encoder-decoder learning method with multi-step reasoning process for open-ended video question answering. We construct a large-scale video question answering dataset. The extensive experiments show the effectiveness of our method.