This paper reports on research conducted at Nippondenso Co., Ltd. and Meiji University on nozzles for heavy duty diesel engines. It focuses on fuel flow analysis in the nozzle, a key component of Fuel Injection Systems (FIS). The optimum design nozzle improves fuel flow and spray characteristics.A newer and tougher emission regulation from the EPA for heavy duty diesel engines will be inevitable from 1998 onward. The goal of every company is to design new FIS in advance which meet the regulations of the future rather than paying for expensive developing costs after new laws have come into effect.To meet the regulation, requirements for FIS are higher injection pressure and injection rate control which create better fuel spray atomization and higher utilization of air. In particular, the nozzle must ensure that high injection pressure is effectively converted to fuel spray without pressure losses.To get higher conversion efficiency from the pressure to the spray for excellent fuel spray atomization, we analyzed and calculated the fuel flow characteristics using a computer model simulation. We also confirmed their effects through experiments using similar enlarged, three - dimensional nozzle models and the actual nozzles.