Abstract: A 3D interactive platform——Virtual Assembly Toolkit aiming at realizing customized Virtual Assembly is presented. New 3D interactive metaphor is applied and 3D interactive primitives in the assembly field are defined and encapsulated in the toolkit. The toolkit supplies programmer with the parser and semantic callback functions about these primitives. It also provides the capabilities of constraint satisfaction and collision detection about workpieces. When a programmer uses it to develop an application study about virtual assembly, the complicated tasks about 3D graphics rendering and interactive primitive manipulation are done by VAT.