The virtual assembly model of mechanical gearbox was researched and its virtual assembly process was realized. By analyzing the information of the virtual assembly object and the relationship between the assembly objects, the virtual assembly model was divided into the object model describing inherent information and dynamic information in the assembly parts, the assembly tree model describing assembly-level information of assembly members, and the relational model describing the parts assembly constraints between the assembly parts. The assembly model information was obtained by the conversion of model data between CAD system and virtual assembly system. Then the assembly model was developed in the virtual assembly system and the implementation of the motion constraints in the virtual assembly process was proposed. By analyzing the DOF based on the space position and geometric constraints, the motion constraints were determined. Using EON and its secondary development tool, the virtual assembly module was built, and then the virtual assembly process of mechanical gearbox was implemented.