Abstract: Concept of variational geometric constraints network (VGCN) is presented, and application of the network to computer aided tolerancing is studied. On the basis of classification of tolerance and fit, three kinds of variational geometric constraints are defined; based on graph theory and TTRS theory, the concepts of mate tree and loop circuit are put forward, and the properties of mate tree and the generation rules of loop circuit are discussed. Using the mate relations between functional surfaces in CAD system, methods for generation of variational geometric constraints network are studied. The scheme is proved effective by applications of the network to generation of tolerance types and dimensional chain.