A cutting insert for milling machining adapted to be mounted on a milling cutter to carry out cutting operation and having primary and secondary cutting edges provided at sides of a rectangular body, each primary cutting edge having a curved line tangential to the adjacent secondary cutting edge, to reduce cutting force and improve flatness of side surface of a machined workpiece, is disclosed. The cutting insert includes an upper face, a bottom face and a plurality of side faces connecting the upper face and the bottom face. The primary cutting edge has chip breakers for guiding chip flow, primary cutting edge land faces and primary cutting edge clearance faces. The primary cutting edges enable a bottom surface and a side surface of a machined workpiece to have a right angle therebetween.