Abstract: The third order nonlinearities in optical materials could induce chaos of the optical field when the material characteristic and laser parameters satisfy some special requirements. In this paper, we start from the Maxwell equations, derive the Maxwell-Bloch equations based on third order nonlinear material, and then transform them into the Lorentz form. Based on these equations, the stable optical field evolution and the chaos are numerically simulated. According to the simulating results, chaos will happen in third order nonlinear material, which leads to unpredictable nonlinear phase shift of the optical field. Such chaos is disadvantageous in suppressing the nonlinear self-focusing in such materials.