Disclosed herein is a master cylinder for a brake system. The master cylinder for a brake system comprises a cylinder body in which a bore is formed, at least one piston provided to be movable forward and backward in the bore, a hydraulic chamber pressurized by the piston, an oil port connected to a reservoir and configured to supply oil to the hydraulic chamber of the cylinder body, and a sealing member installed in the cylinder body and configured to block a flow of oil between the oil port and the hydraulic chamber according to a movement of the piston, wherein the piston includes a piston groove having an inclined surface in contact with the sealing member and a plurality of piston holes formed along an outer circumferential surface of the piston groove configured to communicate the oil port with the hydraulic chamber.