To provide a polyester-based fiber which is improved in abrasion resistance and hydrolysis resistance and further has no smell caused by a free isocyanate compound in operation, melting, or the like; and to provide a polyester composition which can achieve the fiber.

The polyester composition contains a polyester (component A); a thermoplastic elastomer (component B); and a compound (component C) having one carbodiimide group and containing a cyclic structure in which its first nitrogen and second nitrogen are coupled through a linking group, and a fiber comprises the polyester composition.