A semiconductor device includes a semiconductor substrate having a trench defining an active region. A wall oxide is formed on side walls of the active region extending in the longitudinal direction, and an element isolation layer is formed in the trenches. A method of manufacturing a semiconductor device includes forming line-shape first trenches on a semiconductor substrate so as to define an active region; forming a wall oxide on surfaces of the first trenches; forming a second trench which separates the active region into a plurality of active regions; and filling the trenches with an element isolation layer.