Hammer loadable with different sizes of nails, including an adjustment member manually rotatably disposed on the hammerhead. A predetermined number of loading positions are formed on the adjustment member at intervals for loading a nail therein. The loading positions are spaced from the hammering end of the hammerhead by different distances. According to the length of the nail, the adjustment member can be turned to select one of the loading positions for loading the nail therein. The front end of the nail protrudes from the hammering end by a certain length, whereby the tip of the nail can be hammered into the surface of a work piece.