Abstract: A cutting tool holder for a machining center has a body and a fastening device. The fastening device is mounted in the body and has a base, at least two positioning elements and a locking element. The base has an annular wall, a through hole and at least two positioning holes. The through hole is formed through the base. The at least two positioning holes are formed in the annular wall and correspond to the through hole. The at least two positioning elements are located in the at least two positioning holes and engage an annular recess of a seat for a cutting tool to securely yet releasably retain the seat in the base. The locking element is mounted around the base to keep the at least two positioning elements from escaping the at least two positioning holes.