A cutting insert for a machining tool has a structure capable of effectively controlling chips generated on a work-piece. The cutting insert includes an upper surface, a lower surface, a plurality of side surfaces connecting the upper surface and the lower surface and a central hole formed at a central portion thereof. In the cutting insert, a seating surface is formed on a highest portion of the upper surface, a plurality of side cutting edge portions are formed at border portions of the upper surface and the side surfaces. Each side cutting edge portion has a side cutting edge formed at an upper end thereof and a corner cutting edge portion is formed at an intersection of two adjacent side cutting edge portions. The corner cutting edge portion has a cutting edge formed at a front end thereof, a land portion, a downward inclined surface, a corner bottom surface, a first upward inclined surface, a first protrusion, a second upward inclined surface, a second protrusion, an upward grooved surface and the seating surface sequentially formed along a diagonal line directed from the corner cutting edge toward the central hole. The corner bottom surface has the smallest height along the diagonal line.