To reveal the best operating conditions for esterification and transesterification, a series of experiments were performed. The three-dimensional diagrams given in Figure 4 indicate the effects of the reaction time and temperature on conversions of mixed oil to biodiesel by acid catalysts and soybean oil to biodiesel by NaOH. Consideration both of energy consumption and biodiesel conversion, a reaction temperature of 100oC and a reaction time of 90 min were chosen as the optimal operating condition for esterification. However, for transesterification, the favorable reaction conditions were 60oC for 40 min.