A bottom bracket assembly for supporting the crank axle of a bicycle includes a bottom bracket and two bases are securely connected to two open ends of the bottom bracket. Each base has a bearing received in a recess defined in an end thereof. Two collars each have a neck which is inserted and engaged with a central hole of the bearing corresponding thereto and each neck has a tapered inner periphery. An axle extends through the two bearings and the two collars. Two positioning rings are positioned between the axle and the tapered inner periphery of the collar. Each positioning ring has a tapered outer periphery which is matched with the tapered inner periphery of the collar corresponding thereto.