The effect of oil contents on the destruction of polychlorinated biphenyls(PCBs) in transformer oil were investigated under the supercritical water oxidation(SCWO). A specially designed experiments were carried out in the absence and presence of methanol to understand the effect of oil contents on the supercritical water oxidation of PCBs in transformer oil. Experiments were carried out in a continuous flow system at a constant temperature of 500℃, using hydrogen peroxide as oxidant in excess, and different pressure ranging from 24 to 29 MPa. In the absence of methanol, PCBs were destructed after most of oils(98.5% of reduced oils and 37.2 seconds of residence time) were reduced and destruction rates(82.3%) of PCBs were finally achieved at the residence time of 54.9 seconds. On the other hand, in the presence of methanol, the conversions of PCBs were increased by 2 times over that of PCBs in the absence of methanol and PCBs were destructed completely at the residence time of 52.3 seconds.