Polystyrene-polyarylate (PS-PAr) block copolymer was applied as a mold-ability modifier of polycarbonate (PC). From the 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) extraction results, PS-PAr block copolymer was demonstrated to copolymerize with PC via an in situ reaction between the PAr chain and PC. As a result of the chemical bonding between PS and PC chains, the PS dispersed domain in the PS-PAr block copolymer/PC blend could be reduced, on average, to a size smaller than the visible light wavelength. In particular, by adjusting PAr composition to 30 wt % in the fed PS-PAr block copolymer during the melt-mixing process, the PS domain size was completely reduced to be smaller than the visible light wavelength. As a result the blend substrate could satisfy the transparency required for the optical disk substrate with higher memory density: homogeneity under polarizing light, clarity on the reflective plate, and transparency at 400 nm. The melt viscosity could be lowered to the equal viscosity level of PC at about 30 degreesC higher temperature by blending PC 15 wt % with the PS-PAr block copolymer. The lowered melt viscosity could reduce the retardation in the optical disk substrate, which was equivalent to that of the PC substrate processed at 30 degreesC higher. In addition, the PS-PAr block copolymer/PC blend could attain the exact groove transcription at 20 degreesC lower mold temperature than that of PC as a result of the lowered elastic modulus caused by the PS-rich phase. These features of the PS-PAr block copolymer/PC blend indicated a potential to offer an improved process window for the substrate molding. Because of its excellent transparency and a potential for processing flexibility, the PS-PAr block copolymer/PC blend would be a promising material for optical disk substrates with higher memory density.