Abstract: Disclosed is a one-piece expansion nut for a T-joint of pipes, including an internal thread hole at the center thereof and a plurality of long expanding pieces and short expanding pieces alternatively disposed around the outer periphery thereof, wherein the outer diameters formed by the long and short expanding pieces the same, but slightly larger than the inner diameter of one pipe to be jointed, upon putting the expansion nut into the pipe and passing a bolt through another pipe to screw onto the internal thread hole for a T-joint, the long and short expanding pieces being respectively expanded to exert force against the inner wall of the one pipe in two stages, so as to firmly hold the expansion nut in place and effectively complete the T-joint as required.