Virtual Assembly (VA) is an advanced tool for assembly design. Current VA system lacks of intelligent decision support especially in assignment of assembly planning task. There are already some research to enhance the perception ability and intelligent support of VR. However, most of them are focusing on the auto mating or moving navigation, while the study on assembly task is few. The paper analyzed the task in virtual assembly and discussed the system level task as a main topic, which indicated that ‘work-flow’ was a suitably way to implement task aware. Therefore a BDI agent model for assembly task was proposed on the bases of work-flow and the architecture of it was given. After these, the reasoning mechanism was designed to support assembly task awareness.