The effect of molecular weight on the miscibility of polycarbonate (PC) / poly(methyl methacrylate) (PMMA) blend and segregation behavior of PMMA from a homogeneous blend in the temperature gradient are discussed. PC is expected to be used as a substitution of an inorganic glass because of its high impact resistance. However, it is easy to be scratched. In contrast, PMMA has high transparency and marked surface hardness. Therefore, PC/PMMA blend has the ability to be a transparent tough plastic glass with anti-scratch property when PMMA fraction is localized on the surface. The blend of PC and PMMA shows a phase diagram with LCST which is closed to the processing temperature. Therefore, it is not easy to control the morphology at actual processing operations. In contrast, low molecular weight PMMA has better miscibility with PC because of the contribution of mixing entropy. Consequently, a miscible and transparent blend is obtained by melt mixing. Moreover, the segregation behavior to each component from the miscible state is confirmed in this study in the molten state with temperature gradient, which is revealed by FT-IR (ATR). The sample obtained has high concentration of PMMA at high temperature side and vice versa. This will be an important technique to produce an ideal plastic glass.