An office chair includes a frame having a tube with a tapered inner periphery and a plurality of legs extend from the tube. A C-shaped sleeve is movably received in the tube and has a tapered outer periphery. The sleeve has a threaded inner periphery so as to be threadedly engaged with a seat post. Two notches are defined in the tapered outer periphery of the sleeve so that two pins extend through the tube and are engaged with the two notches to restrict the sleeve from being rotated. The sleeve is firmly clamp the seat post when a load is applied downward the seat post. When lifting the seat post, the sleeve is raised and loosened so that the seat post is adjustable.