In this paper the dynamic stresses in a simplified photoelastic model of human mandible which is subjected to impact load in the horizontal direction have been studied by means of the advanced Cranz-Schardin ultrahigh sped camera system. The variation and distribution of the transient dynamic stresses are obtained. The separation of the dynamic principal stresses in the model is completed by solving the dynamic Poisson's equation with the transient dynamic isochromatics and isoclinics of the model combined with the finite difference method. The dynamic mechanical properties of the photoelastic material are also determined by impacting a one-dimensional model with a falling hammer.