A height adjustment mechanism for an adjustable height table includes an outer sleeve, an inner sleeve, a plug, and a locking mechanism. The outer sleeve is disposed outside the inner sleeve, and the outer sleeve and the inner sleeve are slidable relative to each other. The plug is disposed outside the inner sleeve, and is slidably connected to the inner sleeve. The plug is disposed at an upper end of the outer sleeve, and an outer wall of the plug includes a pressing portion. The locking mechanism operates to push the pressing portion tightly against an outer wall of the inner sleeve to increase the friction between the plug and the inner sleeve, so that the plug and the inner sleeve are relatively fixed.