An adjustment wheel assembly of an adjustable stamp, mounted on an adjustment wheel unit of the adjustable stamp, has a printing type cover combined with a torsion spring and pivotally mounted on the adjustable wheel unit, and a fastener mounted on the adjustment wheel unit. The printing type cover has a tab and a snap part formed on another side of the printing type cover to engage the fastener and disengage the fastener when a pressing part of the fastener is pressed. The torsion spring enables the printing type cover to flip up. The printing type cover is closed and fixed by pressing down the tab formed on an edge portion of the printing type cover. Accordingly, users' fingers can be prevented from being stained upon flipping up and closing the printing type cover.