The nonlinear optical properties of cluster WOS 3Cu 3I(2 MePy) 3 by using the Z scan method were studied. The experimental results were simulated based on the steady state solution of rate equations, and the third order nonlinear optical parameters of the cluster were obtained. The theoretical curves are in good agreement with experimental results. The relationship between the sign of the nonlinear refraction index and the ratio of the refrangibilities K r has also been studied. It is demonstrated that when K r>1 the nonlinearity is positive, while K r<1 negative. Numerical results are in consistent with the theory.