A method for manufacturing semiconductor device is disclosed. A substrate with a conductive layer is provided, and a dummy layer is formed on the conductive layer. The dummy layer and at least a portion of the conductive layer are patterned to form several trenches. A first dielectric layer is formed to fill into the trenches so as to form several first dielectric elements in the trenches. The dummy layer is removed to expose parts of the first dielectric elements. A second dielectric layer is formed on the exposed parts of the first dielectric elements, and the second dielectric layer is patterned so that a spacer is formed at a lateral side of each exposed first dielectric element. The conductive layer is patterned by the spacers, so that a patterned conductive portion is formed at each lateral side of each first dielectric element.