A study was performed to correlate the Sauter Mean Diameter (SMD), NOx and particulate emissions of a direct injection diesel engine with various injection pressures and different nozzle geometry.The spray experiments and engine emission tests were conducted in parallel using the same fuel injection system and same operating conditions. With high speed photography and digital image analysis, a light extinction technique was used to obtain the spray characteristics which included spray tip penetration length, spray angle, and overall average SMD for the entire spray. The NOx and particulate emissions were acquired by running the tests on a fully instrumented Caterpillar 3406 heavy duty engine.Experimental results showed that for higher injection pressures, a smaller SMD was observed, i.e. a finer spray was obtained. For this case, a higher NOx and lower particulate resulted. For different nozzle geometry with the same injection duration and injection delivery, a rounded inlet nozzle gave a larger SMD than a sharp-edged inlet nozzle. The change in SMD between the two tips with injection pressure became smaller as the injection pressure increased. Also, a smaller hole size nozzle produced smaller droplet sizes and a smaller amount of particulate; but the trend of NOx depended on the injection pressure. A double injection strategy gave decreased SMD, reduced NOx, and lower particulate at the same time.