The simultaneous effects of pilot fuel quantity and pilot injection timing on engine performance and amount of pollutant emission have been computationally investigated in a High Speed Direct Injection (HSDI) diesel engine. In this study, a modified parameter called “Homogeneity Factor of in-cylinder charge (HF)” has been applied to analyze the air-fuel mixing and combustion processes. For this purpose, the simulated results has been firstly compared with the experimental data and a good agreement has been achieved for simulating the in-cylinder pressure and the amount of pollutant emissions. Then, nine different strategies based on two variables (the amount of fuel mass in pilot and main injection as well as the dwell between two injections) have been investigated. The results show that employing pilot injection results in higher in-cylinder temperature and shorter auto-ignition delay which causes a rapid increase in the rate of NOx formation in early stage of combustion process. It has been found that, pilot fuel quantity has a significant impact on soot emission while its timing could mainly control NOx formation. In addition, by advancing the start of pilot injection timing, the maximum amount of Homogeneity Factor (HF) during pilot injection, is achieved at an earlier time. As a result, for earlier SOP timing, a sufficient mixing is available to achieve a more homogeneous, locally fuel-lean in-cylinder mixture at the time of ignition to form a large portion of premixed mixture. This suggests, by 0.5 mg pilot fuel injection and also retarding the injection timing by 10 crank angle degree, the amount of NOx and soot emissions have been considerably decreased compared to the baseline case. However, increasing the amount of the ISFC (for about 2.5 %) and decreasing the indicated mean effective pressure (for about 2.2 %) should be considered as two negative factors.