Herein, we report the effect of sizing agents on carbon fiber (CF)/epoxy interfacial interaction. In addition to the relationships between sizing agents and composite interfacial properties, which have been the focus of previous studies, this study deals with the effect of sizing agents on the resin impregnation behavior in vacuum assisted resin transfer molding (VaRTM), which affects the productivity of composite manufacturing. For this purpose, CFs treated with different sizing agents were compared. CF surface characteristics were investigated via various analyses. Surface free energy, a parameter of the fiber/matrix adhesion, was measured. Dynamic contact angles were measured to understand the CF wettability. VaRTM tests were performed. Interfacial shear strength (IFSS) of CF/epoxy composites was evaluated via micro-droplet tests. Finally, resin flow characteristics with polymer slip at the CF surface were studied, and the normalized surface free energy, considering the specific surface area, was suggested as the key parameter of IFSS.