Surface treatment of fibers in fiber reinforced polycarbonate injection moldings has been studied. Short glass fibers (GFs) treated with aminopropyltrimethoxysilane (APMS) are bound by a binder (UB) as sizing agent and compounded with polycarbonate using a twin screw extruder (20 wt% GF). Dumbbell specimens with and without weldline are injection-molded for tensile test. Bar specimens are also injection-molded for flexure and Izod impact test. The strengths are greater when a large amount of UB is used in each test. Concentration of UB diffused in the composites is evaluated by microscopic Fourier-Transform Infrared Analysis (FT-IR). The surface condition of GFs treated with APMS influences the diffusion behavior of UB and the concentration varies drastically in the range of 120 mum from the fiber surface. When a large amount of UB is used, the GF is surrounded by a thicker phase containing UB, or 'interphase' which exists between brittle glass fiber and ductile polycarbonate. The interphase that showed ductility contributes to the improvement of the mechanical properties.