Abstract: A pedal-ring tightening structure includes a pedal ring coupled to a supporting central tube of a seat at a predetermined height. At least two clamping elements are coupled to the periphery of the central tube at a predetermined height, such that a sleeve disposed centrally at the pedal ring formed peripherally and integrally with a predetermined width is coupled to the periphery of the clamping elements. A protruding portion extending from an inner rim of a fixing ring is inserted into a space concavely disposed at the upper end of each said clamping element to abut against precisely and press inward each said clamping element coupled to the periphery of the central tube, such that the pedal ring is steadily clamped and positioned at the central tube upon assembly.