Biodiesel prepared by catalyzed mild transesterification has become of much current interest for bioenergy. The ability of a commercial immobilized lipase (Novo Industries-Bagsvaerd, Denmark) from Rhizomucor miehei (Lipozyme IM-77) to catalyze the transesterification of soybean oil and methanol was investigated in this study. Response surface methodology and 5-level-5-factor central composite rotatable design were employed to evaluate the effects on reaction time, temperature, enzyme amount, molar ratio of methanol to soybean oil, and added water content on percentage weight conversion to soybean oil methyl ester by transesterification. Based on ridge max analysis, the optimum synthesis conditions giving 92.2% weight conversion were: reaction time 6.3 h, temperature 36.5degreesC, enzyme amount 0.9 BAUN (Batch Acidolysis Units NOVO), substrate molar ratio 3.4:1, and added water 5.8%.