Abstract In this study, bentonite based catalyst was prepared by impregnation methods in potassium hydroxide solution (35 wt.%). The catalysts were calcined at 500 for 3 h. The best reaction conditions in the presence of KOH/bentonite were determined while modifying the catalyst to soybean oil ratio and the reaction time. Bentonite was proved to enhance the transesterification reaction, which promotes the transformation of KOH to the catalytically active methoxide species. The influence of the operating variables such as reaction time, methanol: soybean oil molar ratio and catalyst concentration on the reaction yield was evaluated. The highest methyl ester yield (94.5%) was obtained for a reaction time of 15 h, a catalyst to soybean oil ratio of 0.6 wt.%, and a methanol to soybean oil molar ratio of 1:12.