The esterification and transesterification of used soybean oil catalyzed by sulfuric acid was studied. The combined effects of temperature, the amount of sulfuric acid as catalyst, and the methanol/used oil molar ratio (MR) on the biodiesel yield were investigated and optimized by using response surface methodology. The optimum operating conditions are T=65 °C, 6.58 wt % catalyst, and 12:1 MR. In addition, the transesterification of used soybean oil at 50, 55, and 65 °C with 12:1 MR and 6.58 wt % of catalyst was performed with samples taken at different times to investigate the effect on the conversion and physical properties. Appropriate functions were fitted to the measured data to relate the progress of the reaction to the physical properties. The refractive index and specific gravity were selected as suitable physical properties to predict the progress of the reaction. In addition, a simple second-order rate equation with Ea=18.5 kJ mol−1 was developed that accurately describes the experimental reaction rate.