A sizing agent for a reinforcement fiber is used for reinforcing thermoplastic matrix resins and spreads on the reinforcement fiber surface and imparts both excellent splittability and bonding performance to thermoplastic matrix resins. The sizing agent includes essentially an ester compound (A) having a vinyl ester group, acrylate group or methacrylate group bonded to at least one of the chain ends of the main chain of the ester compound (A), and of a polyoxyalkylene alkyl ether (B) which is an adduct between an alkylene oxide and a C4-14 monohydric alcohol. When water is added to the sizing agent thereby producing a mixture with a nonvolatile content of 1 weight %, the mixture exhibits a dynamic surface tension ranging from 40 to 55 mN/m determined by the maximum bubble pressure method when bubbles are blown into the mixture at the rate of one bubble per 100 milliseconds.