An integrated circuit device includes a substrate including an active region defined by a device isolation layer, the substrate defining a gate trench extending across the active region, a gate dielectric layer conformally covering an inner surface of the gate trench, and a gate electrode filling the gate trench on the gate dielectric layer. The gate electrode is composed of crystal grains of a single metal, and a diagonal length of at least one of the crystal grains is greater than a height of the active region that is in contact with the gate electrode.