The gasification reaction is very effective not only in the treatment of combustible wastes but to reuse the potential energy generated from the wastes as well as meeting the environmental regulation. The performance of gasification of the combustible wastes was determined numerically by varying the key gasification reaction variables: the oxygen and steam intake relative to the feed-stock. The feed-stocks chosen in this study were waste oil and refined oil. The simulation was performed by changing the combination of key variables. As the oxygen intake was increased, the production of CO and H2 as well as the cold gas efficiency all increased initially, and then they all decreased after reaching a maximum point. On the other hand, the CO2 decreased initially and then increased. As the steam intake was increased, the production of H2 and CO2 increased slowly, however, the production of CO decreased. The optimal O2/fuel ratios for the maximum production of H2 and CO was determined to be 1.0-1.3 and 1.1-1.3 for the waste oil and the refined oil, respectively. The oxygen and steam feed ratios of the waste oil and the refined oil were derived from the assumption that in the gasification the product gas was utilized as fuel and the slag was converted to the environmentally benign slag.