The effects f spray characteristics on engine performance and exhaust emissions in a small direct injection diesel engine were investigated. Spray characteristics of injection into a high-pressure atmosphere were analyzed with high-speed photographs. The ratio of the nozzle length to the diameter was varied from 3 to 7 at the same nozzle hole diameter. The piston cavity diameter and the intake swirl were chosen as other parameters. The effects of such parameters on the brake specific fuel consumption (BSFC), exhaust smoke, nitric oxides (NOx) and hydrocarbon (HC) were discussed. Spray characteristics were regarded as of major importance on BSFC and HC. A smaller piston cavity diameter has lower exhaust smoke, but a higher Nox emission level. The correlation was observed between the spray penetration from each nozzle and the optimum piston cavity diameter for minimizing the BSFC.