In recent years, many oil spill weathering process models have been developed to predict the trajectory and fate of oil at sea. As a part of oil spill damage assessment model, a precise prediction of physical fate and chemical properties of spilled oil on the sea surface is a key factor to assess environmental damages and take response actions. A system dynamic (SD) model is applied to predict the physical fate and properties of spilled oil in this paper, since oil weathering processes affect each other after spillage. The major oil weathering processes, spreading, evaporation, dispersion, emulsification, dissolution, biodegradation, and oil properties, dynamic viscosity, dynamic density, and oil-water interfacial tension, are included in the SD model and simulated later. The main purpose of this paper is to provide a new way to predict oil weathering processes and oil mass balance using the SD model.