A rotary blade for a weed cutter, in which a center face part (3) including an axial center (2) of a thin disc body (1) wholly made of a spiral steel material is in the form of a horizontal plane, and a circumferential part adjacent to the center face part has a one rotation-directional corrugated cutting part (7) having one side face of concave form (4) and the other side face of convex form (5), wherein the cutting part face is corrugated in the circular curved-surface form (8) instead of the triangular form.
