Abstract: A hidden seat tube assembly for a bicycle, comprises: a seat tube; a C ring installed within the seat tube therein; the C ring having an opening so that the C ring has a C cross section; the C ring being welded to an inner wall of the seat tube through at least one welding spot; a retainer arranged at two sides of the opening of the C ring; one side of the retainer having a through hole and another side of the retainer having a threaded hole; a screw unit retained within the through hole and threaded hole of the retainer; by adjusting the engagement position of the screw unit to threaded hole of the retainer, the cross section of the C ring being enlarged or reduced.