A polygonal cutting insert for right angle machining has opposed upper and lower surfaces, a peripheral side surface interconnecting the upper and lower surfaces and having curved first and fourth side surfaces and plane second and third side surfaces, and main and secondary cutting edges alternately formed along the outer peripheries of the upper and lower surfaces. The adjacent main and secondary cutting edges form a right angle therebetween. The first side surface adjoins the main cutting edge of the upper surface. The second side surface adjoins the secondary cutting edge of the lower surface. The third side surface adjoins the secondary cutting edge of the upper surface. The fourth side surface adjoins the main cutting edge of the lower surface. The cutting insert can be mounted on a milling cutter at a positive axial rake angle.