Due to the depleting amount of petroleum reserves and the increasing awareness on environmental concerns, biodiesel has become one of the most demanding and promising substitutes for the petroleum based fossil fuels. In this study, the biodiesel derived from waste cooking oil through the transesterification process was optimized using Response Surface Methodology. The biodiesel derived under optimum conditions was used for investigating the effect of injection pressure and timing on performance, emission and the combustion characteristics of single cylinder, four-stroke direct injection diesel engine at a constant speed of 1500 rpm. On varying the injection pressure and timing, it was found that the combined effect of higher injection pressure of 280 bar and an advanced injection timing of 25.5 degrees bTDC had significant improvement in the brake thermal efficiency, cylinder gas pressure and heat release rate. Reduction in nitric oxide (NO) and smoke emission was also observed. (C) 2012 Elsevier Ltd. All rights reserved.