Abstract: Biodiesel was synthesized with waste oils and methanol in the presence of H3PO4 as catalyst. The reaction conditions were investigated with orthogonal experiment. The optimum conditions are obtained as follows: temperature 70 ℃, the ratio of methanol to waste oils 30∶1, the dosage of catalyst 8%, and reaction time 5 h. The yield of biodiesel reaches 85% under the optimized conditions.