A method for forming a capacitor comprises providing a substrate. A bottom electrode material layer is formed on the substrate. A first mask layer is formed on the bottom electrode material layer. A second mask layer is formed on the first mask layer. The second mask layer is patterned to form a patterned second mask layer in a predetermined region for formation of a capacitor. A plurality of hemispherical grain structures are formed on a sidewall of the patterned second mask layer. The first mask layer is etched by using the hemispherical grain structures and the patterned second mask layer as a mask, thereby forming a patterned first mask layer having a pattern. The pattern of the first mask layer is transferred to the bottom electrode material layer. And, a capacitor dielectric layer and a top electrode layer are formed on the bottom electrode material layer to form the capacitor.