A resin composition includes a fiber and a polyolefin resin and can provide a molded article having excellent mechanical strength such as flexural strength and impact resistance. The resin composition includes (i) a surface-treated fiber (A) which comprises 100 parts by weight of a fiber (A-I) comprising a polyalkylene terephthalate and/or a polyalkylene naphthalene dicarboxylate and 0.1 to 10 parts by weight of a sizing agent (A-II) adhered to the surface of the fiber (A-I), and (ii) a polyolefin resin modified with an unsaturated carboxylic acid and/or an unsaturated carboxylic acid derivative (a modified polyolefin resin (B)) as a resin component.