An armrest of a chair includes a fixing board, a first movable board, two limit plates, a second movable board, a positioning body, a restoring spring, a lining, a foam body, a push button, a bottom cap, and two shading plates. Thus, the horizontal position of the armrest is adjustable in a two-dimensional way, so that the horizontal position of the armrest is adjusted freely and optimally according to a user's practical requirement, thereby providing a comfortable sensation to the user. In addition, the horizontal position of the armrest is adjustable by pressing the push button, so that the horizontal position of the armrest is adjusted easily and quickly, thereby facilitating the user adjusting the horizontal position of the armrest.