Abstract: Workpiece-fixture stiffness and machining errors are studied. Based on the quasi static force analysis of workpiece,a new method to compute the stiffness of workpiece-fixture system is presented. The classic Hertz contact theory and finite element method(FEM) are made use of, yet the contact analysis in FEM and multi-nonlinear calculations are avoided. A model to predict workpiece machining errors based on the deformation of workpiece-fixture system is provided. In this way much of the time of calculation can be cut down.