Abstract: This paper presents a mathematical model and representation model of the assembly．A binary tree is used to represent an assembly in ZD-MCAD Ⅱ．It propposes a constraint based method for describing the mating conditions of the components of an assembly，and the constraints will be translated into a group of non-linear equations that are solved by improving Newton method．The algorithm of assembling and disassembling is also discussed．The method has already been implemented in our feature based parametric system ZD-MCAD Ⅱ．