A quadrangular indexable drill insert includes a substantially flat top and bottom surfaces, a side surface adjoining the top and bottom surfaces, four identical cutting edges of the side surface intersecting the top surface, a through hole extending between the top and bottom surfaces, and a chamfer provided at the bottom edge of the drill insert. Each cutting edge includes a first part edge; a second part edge extending from the first part edge; a third part edge extending away from the central axis, the second and third part edges being joined together by a transition part edge having a concave curved portion and a convex curved portion; and a curved nose portion of predetermined radius disposed at a corner of the insert adjoining the first part edge of one cutting edge with the third part edge of a succeeding cutting edge of the four identical cutting edges.