A vertical MOS transistor includes a substrate, a metal line disposed on the substrate, a semiconductor pillar disposed on and in contact with the metal line, a gate dielectric layer disposed surrounding the semiconductor pillar, a metal gate disposed surrounding a portion of the semiconductor pillar, and a gate electrode disposed in contact with the metal gate. In some embodiments, a width of an end of the gate electrode in contact with the metal gate is narrower than a width of an end of the gate electrode away from the metal gate.