To obtain a high biodiesel yield, the conditions of transesterification of vegetable oil with supercritical methanol are harsh. The reaction temperature is required between 350 and 400 degrees C, and the reaction pressure is required above 20 MPa. To reduce the operating temperature and pressure, many researchers adopted various methods, such as the selection of a new reactor, the addition of a co-solvent, or the usage of microwave irradiation. According to the characteristics of transesterification, a small amount of organic or inorgnic acid was added into the reaction system, and the effects of an acidic condition on transesterification was investigated in this paper. The experiment was conducted in a continuous flow tubular reactor; the temperature ranged from 270 to 350 degrees C; the pressure covered the range from the normal pressure to 15 MPa; the molar ratio of methanol/oil was from 20:1 to 60:1; and the acidity of acidified soybean oil was from 5 to 30 mg of KOH/g. The results showed that the higher yield of biodiesel was obtained at the conditions with the addition of acid compared to the corresponding conditions without acid and the increase of the yield was more remarkable under lower pressure and temperature correspondingly. Besides, it was found that the effect of inorgnic acid on transesterification was more obvious than that of organic acid. The yield of biodiesel could reach 91.4% when the reaction temperature, the reaction pressure, the methanol/oil molar ratio, and the acidity of acidified soybean oil were 350 degrees C, 15 MPa, 40:1, and 15.0 mg of KOH/g, respectively. It was also revealed that the transesterification reaction was still controlled by reaction kinetics to a great degree, although the reaction had reached the supercritical status.