In this paper, the phenomena of a test particle diffusion near a single magnetic island in a Tokamak is investigated by the method of Monte Carlo and an effective diffusion time is used to describe the effect of diffusion by the island quantitatively. The results show that there is a uniform function between effective diffusion time and a value called collision raie. The relationsipe between the effective diffusion time and the collision rate and the particle density distribution inside and near the islnad are calculated.