In this paper, the effects of two factors named plunge depth and translational speed on tensile strength, flexural strength, and hardness of polycarbonate reinforced with Al2O3 nanoparticles by friction stir processing (FSP) was investigated. To study the effects of the mentioned variables, design of experiments (DOE) and statistical analysis were used. Each of the factors was considered at five levels. A total of 14 specimens were subjected to FSP. It was found that the effect of plunge depth on the values of tensile strength, flexural strength, and hardness of the FSP-ed samples was more than the translational speed. Increasing the plunge depth from 0 to 0.4 mm caused a 52.3% decrease in tensile strength and a 42.6% decrease in flexural strength. In addition, it was found that increasing the plunge depth had a stronger effect on the tensile strength and caused a more severe decrease in it. According to the statistical analysis, the optimal plunge depth and translational speed to create the highest tensile strength and flexural strength is 0 mm and 42–46 mm/min, respectively. The hardness of the processed sample was inversely proportional to the plunge depth and translational speed. By increasing the plunge depth from 0 to 0.4 mm and increasing the translational speed from 30 to 90 mm/min, the hardness of the processed zone decreased by 38.8% and 32.6%, respectively.