Virtual reality is now becoming matured for serious engineering applications. This paper describes VDVAS

an integrated multi modal virtual design and virtual assembly environment

within which the designers are enabled to create both the solid model of a part and the assembly model of a product in 3D space by direct 3D manipulations and voice commands intuitively and easily

to perform assembly planning interactively in 3D space

and to simulate and check assembly process. One important feature of VDVAS lies in that it allows designers to modify components of an assembly(if any design flaw is found) during in the process of assembly modeling and simulation without the need of time consuming data exchange between virtual environment and other CAD applications. This is

of course

achieved by integrating virtual assembly with virtual design. Experiments show that designers can perform assembly planning and verification in VDVAS as if they were

to some extent

in the real physical world.