This study aimed to develop an optimal procedure for the transesterification of soybean oil to biodiesel catalyzed by an ionic liquid (IL). Response surface methodology (RSM) based on the central composite rotatable design (CCRD) was used to optimize the variables, including the methanol/oil molar ratio, the amount of IL, and the reaction time for the transesterification of soybean oil at 120 degrees C. The optimum conditions for the transesterification of soybean oil were as follows: n(soybean oil)/n(methanol) 14:1, amount of catalyst 4wt% to soybean oil, reaction time 8.7h, and reaction temperature 120 degrees C. Under the optimized conditions, the yield of biodiesel reached 94.8%, in close agreement with the values predicted by the mathematical model (95.8%). The results show that the RSM based on CCRD is adaptable for biodiesel yield studied in the current transesterification system. The [(CH3CH2)(3)N(CH2)(3)SO3H]HSO4 IL showed good catalytic activity and high stability among the IL used in the transesterification of soybean oil, and could be reused six times without noticeable drop in activity.