In this study, direct decomposition of waste oil was attempted by use of plasma-assisted combustion under reduced pressure. Experiments were performed to compare the emission spectrum, composition and production of the gases generated by plasma-assisted combustion and reduced-pressure combustion. It was found that plasma-assisted combustion increased the total emission intensity, and that OH emission intensity was increased more than 30 times. In addition, H(2) and CO increased, whereas CO(2) decreased in the generated gas. This study showed the decomposition characteristics of waste oil during plasma-assisted combustion under reduced pressure, and it indicated the possibility that it can be used as a new waste-treatment technology.