The present invention discloses a cutting insert for a machining tool having a structure being capable of controlling effectively chips generated on a work-piece. The cutting insert according to the present insert has 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, and is characterized in that a flat seating surface is formed on a highest level of the upper surface; a plurality of side cutting edge portions are formed at border portions of the upper surface and the side surfaces; a corner cutting edge portion is formed at a border portion 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 protrusion, an upward inclined surface and the seating surface are formed on the corner cutting edge portion sequentially along a diagonal line directed from the cutting edge to the central hole; a pair of second protrusions are formed and disposed symmetrically on both sides of the diagonal line so that the corner bottom surface is surrounded by the downward inclined surface, the first protrusion, and the second protrusions; and the corner bottom surface has the smallest height and the seating surface has the largest height along the diagonal line.