Polycarbonate (PC) is widely employed in industrial fields because of its excellent transparency, mechanical toughness and heat resistance. However, its Young’s modulus is low as compared with other plastic glasses, leading to low surface hardness. Therefore, the modulus enhancement is required to widen applications. We demonstrate new methods to enhance the modulus of PC without losing its advantages by blending a specific metal salt or poly(methyl methacrylate) with low molecular weight (L-PMMA). The modulus enhancement is achieved by a small amount of lithium perchlorate (LiClO4) without losing its transparency, mechanical toughness and heat resistance, which is significantly different from a conventional antiplasticized system. The electrostatic interaction between PC chain and Li cation is responsible for the phenomenon. The modulus enhancement is also detected by adding L-PMMA. Furthermore, the blend shows segregation behavior during injection-molding. Consequently, L-PMMA is localized at surface of a transparent product, which improves the surface hardness effectively. These methods must be available in industry to enhance the mechanical properties of PC.