When Zinc diffuses into undoped InP, the diffusion profiles are severely influenced by the process parameters, such as the diffusion temperature, the diffusion time, etc. In order to reduce the surface damage and enhance reproducibility, the diffusion temperature and the diffusion time are optimized. Under optimized. diffusion temperature, curve of diffusion depth versus the square root of the diffusion time is achieved. From this curve, the diffusion coefficient for zinc under the optimized temperature is calculated. The zinc profile was determined by electrochemical capacitance-voltage profiling (ECV), according to which zinc diffusion mechanism was explained.