A carbon fiber sizing agent imparts good bonding performance to carbon fiber, is used to reinforce a thermoplastic matrix resin, and provides a carbon fiber strand applied with the sizing agent and a fiber-reinforced composite reinforced with the carbon fiber strand. The sizing agent for carbon fiber is used to reinforce thermoplastic matrix resin. The sizing agent essentially contains a polymer component having a glass transition temperature of at least 20 deg.C. and exhibits no endothermic peaks indicating an endothermic heat of fusion due to crystalline melting of at least 3 J/g in a determination with a DSC. The weight ratio of the polymer component is 10 to 100 wt % of the nonvolatile components of the sizing agent. The polymer component is at least one component selected from the group consisting of an aromatic polyester resin, aromatic polyester-polyurethane resin and amine-modified aromatic epoxy resin.