BACKGROUND: The synthesis of betulinic acid ester using betulinic acid and oleyl alcohol catalyzed by Novozym 435 (immobilized Candida antarctica lipase) was carried out. Response surface methodology (RSM) based on a five-level, three-variable, central composite rotatable design (CCRD) was employed to evaluate the interactive effects of various parameters. The parameters were reaction time (8-16h), temperature (20-60 degrees C) and enzyme amount (120-160mg). RESULTS: Simultaneously increasing reaction time, temperature and amount of enzyme increased the yields of betulinic acid ester produced. CONCLUSION: The optimum conditions derived via RSM for the reaction were reaction time of 10.2 h, temperature of 53.1 degrees C and enzyme amount of 138 mg. The actual experimental yield was 48.5% under optimum conditions, which compared well with the maximum predicted value of 47.6%. (c) 2008 Society of Chemical Industry.