In numerical simulation for oil transport at sea surface, we couldn't have known how to decide horizontal turbulent diffusion coefficients of oil. In this study we made a diffusion model at sea surface to predict oil turbulent diffusion coefficient from the results of drift experiments using pseudo oil made with sponge rubber in real sea. Diffusion coefficient at sea surface is larger than under water. Diffusion coefficient doesn't depend on wind velocity and current velocity in these experimental conditions. We conducted numerical simulations using derived diffusion model. The result using derived diffusion model is better than it using traditional method in initial stage of oil spill. Derived diffusion model is very plain, therefore it is easy to incorporate the diffusion model in the other oil transport simulations.