A method for manufacturing the DRAM includes first providing a substrate where patterned first mask layer and deep trenches exposed by the patterned first mask layer are formed. Deep trench capacitors are formed in the deep trenches and each of the deep trench capacitors includes a lower electrode, an upper electrode, and a capacitor dielectric layer. A device isolation layer is formed in the first mask layer and the substrate for defining an active region. The first mask layer is removed for exposing the substrate, and a semiconductor layer is formed on the exposed substrate. The semiconductor layer and the substrate are patterned for forming trenches, and the bottom of the trench is adjacent to the upper electrodes of the trench capacitor. Gate structures filling into the trenches are formed on the substrate. A doped region is formed in the substrate adjacent to a side of the gate structure.