An adjustment device for vertically adjusting a chair headrest is revealed and is installed on a seat back frame while in use. The adjustment device includes a sliding member, a fixing member and a button set. A vertical toothed part of the button set is moved and separated from a first rack of the sliding member when the button set is pressed. Then the sliding member is moved vertically relative to the fixing member for synchronous adjustment of vertical position of the headrest. After the button set being released, the vertical toothed part of the button set is turned back and engaged with the first rack of the sliding member. Thus the sliding member and the headrest are positioned at a new position after the adjustment.
