Abstract: A seat supporting assembly of a bicycle, comprises a seat tube having a shape as a wing of a plane; a seat rod received in a hollow shape of the seat tube; the seat rod having a shape as a plane wing corresponding to that of the seat tube; the seat rod having a first inclined surface and a through hole; a tightening assembly having a tightening unit and a stud; the tightening unit having a second inclined surface, an outer periphery and a stud hole; the second inclined surface having a shape corresponding to that of the first inclined surface; the outer periphery of the tightening unit being matched to an inner wall of the hollow space of the seat rod; the stud hole is corresponding to the through hole of the seat rod. The seat tube has the effect of reduction of the rotation of the seat rod.