A seat support structure of a bicycle comprises a seat supporting tube; a seat supporting rod received in the seat supporting tube; a lower lateral side of the seat supporting rod having a first inclined surface; the seat supporting rod being formed with an upper hole and a lower tapered hole; a tightening assembly received in the seat supporting tube; the tightening assembly having a tightening unit, an embedding unit; and a stud; in assembly, a screw post of a stud passes through the upper hole, the lower tapered hole, the stud hole of the tightening assembly and the embedding hole of an embedding hole so as to combine the tightening assembly to the seat supporting rod; then the seat supporting rod and the tightening assembly are placed within the seat supporting tube.