Virtual assembly is one of the most important applications of virtual reality in product design fields. Virtual assembly method based on physically modeling is proposed for further investigating the kinematics and dynamics performance of assembling parts during virtual assembly process in this paper. The automatic constraints mapping method

which maps geometrical constraints implied in assembly relationships onto joint relationships based on degree of freedom analysis

is presented to construct the physical constraint models for physically simulation in virtual assembly environment. A variable stiffness spring model

which maps the geometrical displacement onto assembly force

is devised to realize the interactive input of assembly force. The methods are implemented in the development of VIRDAS (Virtual Reality Design and Assembly System) and the primary process of virtual assembly based on physically modeling in VIRDAS is also given in this paper.