The effect of second phase size (200, 400, 600 and 800 nm) and volume fraction (10, 20 and 30%) on light transmission behavior of ZnS/diamond composite was simulated using Mie theory and FDTD method. It is expected that the results of FDTD method to be close to experimental results. To validate this approach, simulation results of Mie theory andFDTD method were compared with experimental light transmission (T) of 88.7–11.3 vol% ZnS/diamond composite, reported previously by Xue and et al, and it is shown that results ofFDTD method matched well with these experimental results. Because of considering particles interaction and the higher accuracy of theFDTD method in calculating the scattering cross section than the Mie theory, the difference between simulation curves of ZnS/diamond nanocomposite based on Mie theory and FDTD method became more significant with increasing second phase size particularly at greater volume fractions and at short wavelengths. Simulated light transmission curves obtained by FDTD method showed that the optimized second phase size should be smaller than 800 nmfor volume fraction of 10% and smaller than 200 nmfor volume fractions of 20 and 30%. Onthe other words, within these sizes, the reduction of light transmission of ZnS/diamond composite compared to monolithic ZnS is lower than 10%.