In this paper, a novel method based on non-uniform rational B-spline (NURBS) curves is proposed to simulate a physics-based muscle model for facial expression animation. In our system, non-uniform rational B-spline curves are used to construct facial muscles based on the anatomical knowledge. By changing the weights of the NURBS curves, we can form a motion vector to control facial muscle movements. With different number of control points, muscles are more controllable so that more detailed facial expression and mouth shapes can be simu쬡ted. Our approach can simulate different types of muscles including linear and sphincter mus죬es, as well as the non-linear part of the face like fatty tissue.