The present work focus on the use of waste cooking oil in diesel engine. A single cylinder four stroke diesel engine of Kirlosker make was run with waste cooking oil and its blends of B10, B20, B30, B40 and B50 to evaluate the performance with regard to brake power, brake thermal efficiency, brake specific fuel consumption and emissions such as NOX, UBHC, CO and CO2. For comparison, the same parameters were determined for engine operation with conventional diesel oil also. The effect of injector opening pressure (IOP) on the performance and emission characteristics of biodiesel blends of B10, B20, B30, B40 and B50 at three different IOP of 180, 200 and 220 bar were studied and the optimum biodiesel blend was determined which has resulted in better performance and emission characteristics with biodiesel blends as fuel. Also, investigations were carried out to study the effect of ethanol blend with B20 biodiesel blends on the performance and emission characteristics at optimum IOP of 220 bar for the best ethanol blend was determined. The result of this investigations revealed that the performance of engine with biodiesel and its blends has been improved at higher injection pressure. At an IOP of 220 bar, the performance and emission characteristics of biodiesel blends of B20 were better as compared to 200 bar, which is the normal IOP for diesel operation. The results of the studies on effect of blends at an optimised IOP of 220 bar has indicated that the B20 blend gave better performance and produced lower exhaust emissions from the engine. The biodiesel substantially reduces unburnt hydrocarbons, carbon monoxide and CO2 in exhaust gases, and slightly higher NOX than diesel fuel, but it was found that there is no significant difference between diesel and biodiesel in thus respect. Also, the result of the studies on effect of ethanol on optimized biodiesel blend revealed that B20E10 blend gives better performance and lower exhaust emissions. The blend B20E10 gives better performance and lower emission of UBHC, CO and CO2 than the B20 blend at IOP of 220 bar.