Abstract: An improved rear hub transmission mechanism for use in bicycles including a hub body, an axle, a sleeve, and a driving assembly. The hub body has a through hole and an axle assembly. The through hole is circumferentially provided with a plurality of grooves and has a polygonal hole and a round hole at either ends thereof. The sleeve has a lower section having external threads and a bracing section, and an upper section having a plurality of grooves and a flange. The grooves in the sleeve lock with the grooves in the through hole while the flange fits in with the polygonal hole of the through hole when the sleeve is inserted into the through hole. The driving assembly consists of a mounting, a plurality of resilient members and retaining elements, and a gear seat. One end of the mounting is provided with a tube for insertion into the round hole of the through hole for positioning the sleeve.