This paper is to investigate the optimization of mechanical properties for the maximum tensile strength, elongation, and impact strength of Polycarbonate and Polymethyl methacrylate (PC/PMMA) blends by injection molding process. The PC/PMMA plastics composites with different blending percentage are first blended have been injected as the tensile and impact specimens designed according to ASTM, type V by injection molding machine. Taguchi’s method is then used to find the optimal parameters for the maximum tensile strength, elongation and impact strength. The control factors selected in this study are melt temperature, packing pressure, mold temperature and cooling time. An ANOVA table has been used for determining the significance of injection molding parameters. Results of experiments show that the melt temperature is the most significant parameter for improvement of mechanical properties of PC-PMMA plastics composites. Blends with high PC concentrations result in low tensile strength and high impact strength. Illumination testing of the tatol internal reflection (TIR) of PC/PMMA blends has been proceeded and the TIR lens illumination intensity is compared with three compositions of the PC/PMMA blends. Illumination results show that the PC/PMMA 80/20 blend has the highest intensity of illumination. Results of this study can be applied on the optimization of injection molding parameters for polymer blends of LED lens.