The present work deals with statistical optimization of a direct-injection diesel engine. Due to the NOX and particulate tradeoff relation in diesel engines it is difficult to reduce these two pollutants simultaneously. Previous studies have indicated the effects of inlet air components and injection nozzle opening pressure on NOX and particulate emissions.The objective of this study was to find the optimized operating parameters point for a single-cylinder DI diesel engine at 1500 rpm and 75% load to reach a desired NOX, smoke and fuel efficiency levels, using a Response Surface Method (RSM). The operating parameters used in the optimization process are inlet air CO₂% (simulation of EGR) and injection nozzle opening pressure. A mathematical statement of goal was defined in the form of an objective function.It was shown that RSM is an effective method to obtain the optimum operating parameters point. RSM optimization resulted in simultaneous reduction in NOX and particulate emissions without sacrificing fuel efficiency. Effects of operating parameters on emissions were noted and discussed in the process of optimization.