The virtual teleconferencing system provides a collaborative virtual environment for applications such as collaborative seminar. The challenge of the natural interaction is for the participants to be aware of the virtual environment. A video avatar is a computer-synthesized three-dimension image created using live video. We study the factors contributing to the participant's natural interaction and introduce a video avatar based participant model to implement the natural, accurate and convenient interaction among participants. Our virtual teleconferencing prototype provides a seamless virtual collaboration work environment including eye contacts and gaze awareness. Our experiments on a three parties collaborative seminar show that natural interaction synthesizing can significantly improve the communication and that natural multi-model human computer interaction is necessary in collaboration.