An experimental expression was proposed as effective means for determining the optimal nozzle diameter. To obtain the expression, the cylinder pressure was measured and recorded in a combustion analyzer. The pressure data of 80 cycles were averaged. The fuel line pressure sensor was mounted in the inlet nozzle holder, and the needle lift was sensed by the variation of inductive current. The proposed expression based on experimental data was used for determining the nozzle diameter in direct injected engines. The validity of the proposed expression was identified with the experimental data from five different diesel engines. The results showed that the values from the expression agreed with the actual nozzle specification at medium and high engine speed. The nozzle diameter in diesel engines as a function of power and specific fuel consumption could be predicted. Also, the injection duration could be found as a terms of power, specific fuel consumption, and determined nozzle diameter.