Waste or used transformer oil (UTO) possesses a considerable heating value and physical properties similar to diesel fuel. But, the viscosity and density are marginally higher than diesel fuel. The present investigation is aimed to study the combustion, performance and emission characteristics of a single cylinder, four stroke, air cooled DI engine fueled with UTO at different nozzle opening pressures varying from 200bar to 250bar at a regular interval of 10bar. The results of the investigation showed that operating the engine fuled with UTO at 230bar nozzle opening pressure gave a higher brake thermal efficiency, higher nitric oxide (NO) and lower smoke emissions than that of UTO at 200bar which is a standard nozzle opening pressure set by the manufacturer. The maximum heat release and the maximum cylinder pressure at 230bar are found to be higher than that of UTO at 200bar. In comparison with diesel operation, UTO at 230bar operation gave higher NO and lower smoke. The results are presented in this paper.