The methyl ester content decreased with the increase in the used frying oil content in the oil feedstock. This similar trend was also obtained for all mixed oil feedstocks. In addition, it was observed that the light gelatinous precipitate(soap) and the emulsion between the biodiesel and washing water layers were occurred during the reaction and washing step, respectively, when the used frying oil and mixed oils were used as feedstocks. The amount of the gelatinous precipitate increased with further increased in the used frying oil content in oil feedstock. On the other hand, in the case of fresh vegetable oils, the light gelatinous precipitate and the emulsion formation were not observed during the reaction and washing step, respectively.