The process for the transesterification of high-acidity waste cooking oil catalyzed by conc.H2SO4 was studied first. The optimal conditions were found to be reaction time of 5 h, reaction temperature of 60 degrees C, the dosage of conc.H2SO4 18.7%, and molar ratio of methanol to oil 9:1. Then sodium methoxide was used as a catalyst in preparing biodiesel. The optimal conditions were obtained after reaction at 65 degrees C for 2 h when 0.8% of sodium methoxide and 7:1 mole ratio of methanol to waste oil was used. The biodiesel obtained may reach the ASTM standards.