Abstract: A molded plastic cap buckle and adjustment strap are provided for adjusting the circumference of the head band of a cap to adjust the cap to the head of a user. The buckle is secured to one end of the head band of the cap and the adjustment strap is secured at the other end of the head band. The buckle is made from a unitary member having parts hinged together along an elongated axis that lies generally parallel to the longitudinal axis of the adjustment strap. The buckle and strap have cooperating recesses and projections for securing the buckle and strap in selected adjusted position. The recesses and projections are constructed to provide for one-way longitudinal movement of the strap relative to the buckle in one direction when the buckle and strap are assembled to reduce the effective circumference of the head band, but preclude longitudinal movement in the opposite direction.