A replaceable decoration unit for an accessory has an accommodating element, an outer cover and an outer decorating element. The decoration unit can be used in accessories such as earrings, watches, pendants, necklaces, waistlets, bracelets or key chains. When the outer cover is covered on the accommodating element, the outer decorating element is clamped and positioned between the outer cover and the accommodating element. The user can change the outer decorating element depending on usage demand. To change the outer decorating element, the user can open the outer cover and then remove the outer decorating element for replacement, thereby varying the look and style of the accessory. Besides, the outer decorating element can be made of solid materials such as metal or plastic, or soft materials such as fabric or leather, or the combination of the solid materials and the soft materials.