An object of the invention is to develop fibers which include an olefin polymer and are suitable to form a nonwoven fabric, and to develop such nonwoven fabrics. The invention provides a fiber including an olefin polymer composition including 100 parts by weight of an olefin polymer and 0.1 to 30 parts by weight of an acid-form anionic surfactant, the acid-form anionic surfactant having been melt kneaded in the fiber.