Characteristics of the transesterification reaction between triglycerides in soy bean oil and methanol were investigated in the presence of acid catalysts. such as sulfuric acid and PTS (p-toluene sulfonic acid). Concentrations of diglyceride and monoglyceride which were intermediates in the reaction mixtures, were far below 10% of triglyceride under any reaction conditions. Thus, conversion of the reaction could be determined from the concentration of triglyceride. Dried PTS had more superior catalytic power than sulfuric acid for transesterification reaction between soy bean oil and methanol. When transesterification reaction of soy bean oil was catalyzed by 1 wt% of PTS at methanol stoichiometric mole ratio of 2 and $65^{circ}C$, final conversion reached 95% within 48 hours. If FAME (fatty acid methyl ester) was added into reaction mixture of soy bean oil, methanol and PTS catalyst, it converted reaction mixture into homogeneous phase, and substantially increased reaction rate. When reaction mixture was freely boiling which had equal volumetric amount of FAME to soy bean oil, methanol stoichiometric mole ratio of 2 and 1 wt% of PTS, final conversion achieved value of 94% and temperature approached to $110^{circ}C$ within 2 hours.