To obtain the subject sizing composition containing a polymer of specific monomers, capable of improving wettability of reinforcing glass fibers for FRP and clarity of FRP.

This sizing composition comprises a low-molecular weight polymer (effective for clarity) is obtained by subjecting (a) (meth)acrylic acid, (b) a 1-8C alkyl ester, glycidyl ester or a 1-4C hydroxyalkyl ester of acrylic acid or methacrylic acid, (c) a vinyl ester of a ≤10C saturated aliphatic monocarboxylic acid (e.g. vinyl acetate, etc.), (d) (meth)acrylamide, its N- substituted derivative or vinyl ether, (e) an allyl (or methallyl) saturated aliphatic monocarboxylate, (f) itaconic acid and/or maleic acid and/or fumaric acid diester and (g) a monomer such as styrene, etc., as monomers to emulsion polymerization and has <0.4 dl/g intrinsic viscosity. The polymer is formulated with a silane coupling agent, a lubricant, acetic acid, etc., to give the objective sizing composition. A reinforcing glass fiber is treated with the sizing composition to effectively improve wettability of the glass fiber.