A check valve for a pump of an electronically controlled brake system includes a valve body provided with a fluid path, a spring seat placed in an outlet of the fluid path of the valve body, a valve rod fitted at a lower end thereof into an inlet of the fluid path of the valve body, a valve piston provided with a through hole that is selectively opened and closed by an upper end of the valve rod, as the valve piston moves upward and downward along an inner wall of the valve body, and a spring interposed between the spring seat and the valve piston.