A vinyl-functionalized carbon nanotubes (CNTs)-containing vinyl ester sizing agent was prepared for improving the interlaminar shear strength (ILSS) and impact properties of carbon fibers (CFs) reinforced unsaturated polyester (UP) composites. More reactive unsaturated double bonds and better interfacial compatibility make vinyl ester R806 a more suitable resin for preparing sizing agent for CF/UP composites when compared to the commercial epoxy sizing agent. The surface characteristics of CFs and the interfacial properties of the composites before and after sizing agent treatment were investigated. The observed uniformly dispersed CNTs-containing sizing agent on the CFs surface obviously increased the surface roughness. The amount of polar functional groups and the wettability of CFs were significantly enhanced after the coating treatment. The ILSS and impact toughness were enhanced by 28.4 and 53.3 %, respectively. The sizing agent effectively enhanced the interfacial adhesion by improving the surface energy, and increasing chemical bonding and mechanical interlocking.