A method for fabricating a capacitor in DRAM. A horizontal buried doped region is formed in a substrate. A pad oxide layer and a mask layer are formed in sequence on the substrate. A plurality of first trenches is formed in the substrate. Thus, a plurality of bit lines is formed in the substrate. A plurality of second trenches is formed in the substrate to expose the surface of the bit lines, wherein the second trenches cross the first trenches. Thus, a plurality of silicon islands on the bit lines is formed. A first insulation layer is formed in the first trenches and the second trenches, wherein the sidewall of the silicon islands are partly exposed and doped regions are formed in the exposed sidewall. A gate oxide layer is formed on the sidewall of the silicon islands. A spacer is formed on the gate oxide layer. A second insulation layer is formed over the substrate. The mask layer and the pad oxide layer are removed to expose the top surfaces of the silicon islands. A patterned conductive layer is formed over the substrate. A dielectric layer and an upper electrode are formed in sequence over the substrate.