Abstract: Cubic Boron Nitride (CBN) cutting tools have been found to exhibit brittle fracture serious during machining alloy cast iron(HRc 58).This pa per introduces the results of metal cutting and photoelastic tests performed to determine optimum rake angle, in accordance with the stresas distribution analyzed by using the theory of elasticity.The theoretical analysis is basical agreement with the experimental results.The maximum stress point moves to compressive stress, depended on the experimental data, when negative rake angle substituted for positive one and other cutting conditions kept fixed. Besides, tool life is longer and fracture of tools is improved as well.On stress analysis and experiments, the optimum rake angle is defined and necessity of negative first face land has been discussed briefly for brittle tools.