An expander nut is for being assembled to an upper end of a steering tube of a bicycle. The expander nut includes a base nut, two wedge halves having interlocking tapered surfaces and assembled to the base nut, a top cap mounted on the wedge halves, and a bolt screwed to the base nut for controlling the wedge halves to slide relatively along the tapered surfaces so that the wedge halves move outward to wedge an inner surface of the steering tube.