A kind of model oriented assembly process simulation in WTK environment has been proposed. In the model, firstly, the assembly hierarchy of mechanical products can be presented clearly. Secondly, the assembly constraints between parts can be added to the model easily. At last, the assembly process information such as assembly sequence and path for each part can be recorded at real time in the model. Then, the converted method from PRO/E system to WTK environment about the mechanical product’s design information including geometric models, product design hierarchies and constraints has been discussed, which makes for easier building the model in the virtual environment.