Response surface methodology (RSM) was successfully applied to optimize transesterification of biodiesel from soybean oil catalyzed by ionic liquid. The effects of various reaction conditions, including reaction time, methanol/oil molar ratio and the amount of ionic liquid were investigated. A central composite design (CCD) was employed to search for the optimal yield of biodiesel. The obtained optimum conditions were n(soybean oil) : n(methanol) =1:13, amount of catalyst of 4 wt% to soybean oil and reaction time 8.5 h at 120
. Under the optimized conditions, the yield of biodiesel reached 94.7 %, in close agreement with values predicted by the mathematical model. The [HSO3-pmim]HSO4 ionic liquid showed the best catalytic activity, good stability on the transesterification of soybean oil and could be reused six times without noticeable drop in activity.