Abstract: In this paper，the concepts of virtual representation and instance representation of assembly constraints，which are employed in our feature based parametric modeling system ZD-MCADII，are brought forward．The ZD-MCADII assembly modeling functionality can be used to model assemblies quickly，using mating relations like against，insert etc．These constraints are translated into a group of non-linear equations that are solved by improving Newton method．It also proposes a concurrent assembly design model based on the assembly template，in which the major model of parts is described．As a result，ZD-MCADII has a capability to support the design with a mixed top-down and bottom-up design mode．