The surface treatment processes of carbon fibers is very important, because of their significant impact on fiber handling, filament protection, and interfacial properties. In this study, the effects of two different sizing agents with different molecular weights, with or without a nonionic surfactant, on the performance of a melt-spun polyacrylonitrile-based carbon fiber and carbon fiber/epoxy interfacial adhesion are investigated. The focusing property and spread-ability of a low-molecularweight sizing agent with a surfactant show outstanding performances because of the high penetration between the fibers and high interfacial bonding with the fibers. In addition, wettability of the matrix (epoxy resin) of the low-molecular-weight sizing agent are superior to those of the high-molecular-weight sizing agent. Furthermore, the nonionic surfactant used as an assistant improves the sizing amount and wettability by forming micelles with the epoxy. The interfacial shear strength (IFSS) of the low-molecular-weight sizing agent with a surfactant is also superior to that of other sizing agents. The IFSS is closely related to the sizing amount of the coating on the carbon fiber surface and matrix wettability.