The main objective of this investigation is to reduce the NOx emission of a stationary diesel engine with less sacrifice on smoke intensity and brake thermal efficiency (BTE). Fuel injection timing, percentage of EGR and fuel injection pressure are chosen as factors influencing the objective. Three levels were chosen in each factor and design of experiments method was employed to design the experiments. Taguchi's L9 orthogonal array was used to conduct the engine tests with different levels of the chosen factors. Test results were analysed by analysis of variance (ANOVA) method and ANOVA table was formed for each response variable. From the ANOVA table the most influencing factor and also the significance of each factor affecting the NOx emission, smoke intensity and BTE was found out. Response graph was drawn for each response variable to determine the optimum combination of the factor levels. This optimum combination was confirmed experimentally. © 2011 Inderscience Enterprises Ltd.