Direct decomposition of PCBs-containing insulation oil as a model of persistent liquid waste was attempted by use of microwave non-thermal plasma. The decomposition ratio of insulation oil was found to be about 80%, and the dechloridation ratio of trichlorobenzene was 78-87%. In the case of PCBs, although about 5.2 x 10(-4)% of PCBs were converted to PCDFs and the decomposition ratio of non-ortho Co-PCBs was 3.0-46%, the total decomposition ratio was 77%. As a result, PCBs-containing insulation oil in a liquid state could be decomposed by use of microwave non-thermal plasma.