A numerical optimization technique has been applied in order to obtain the optimum level of some design variables of the diesel fuel injection nozzle. A description of the optimization procedure is provided together with a discussion of the physical and mathematical flow model used. Sample applications of the optimization procedure addressing design of a fuel injection nozzle consideration along with comparisons among three different fuels are presented.The fuel injection mean rate, the diameter of the injection cavity, and the diameter of the injection orifice were used as the objective functions in different aspects of the optimization procedure.Chosen design variables were: the angle of the needle tip, the diameter of the nozzle injection cavity, and the injection orifice diameter. Some constraints characterizing the operating conditions of the injection nozzles were taken under consideration. In order to assess the influence of the fuels' physical properties on the chosen design parameters of fuel injection nozzles, the analysis was carried out for fuels with different characteristics. The diesel fuel was selected as a reference fuel, in addition, neat sunflower oil and 50/50 mixture of sunflower and diesel fuel were used.The analysis indicated the possibility of significant reduction in the diameter of the injection cavity and the diameter of the injection orifices. It was evident that the physical properties of fuels have significant influence on the selected design variables. The necessary changes in design of the injection nozzle is recommended before long term operation on the fuels with different physical properties.