A memory device is provided. The memory device includes a substrate, a trench having an upper portion and a lower portion formed in the substrate, a trench capacitor formed in the lower portion of the trench, a collar dielectric layer formed on a sidewall of the trench capacitor and extending away from a top surface of the substrate, a first doping region formed on a side of the upper portion of the trench in the substrate for serving as source/drain, a conductive layer formed in the trench and electrically connected to the first doping region, a top dielectric layer formed on conductive layer, a gate formed on the top dielectric layer, an epitaxy layer formed on both sides of the gate and on the substrate and a second doping area formed on a top of the epitaxy layer for serving as source/drain.