A drill structure includes a shank part and a flute part arranged on one end of the shank part. A chisel edge is formed on the front end of the flute part and two primary relief surfaces with tile directions toward the shank part are symmetrically formed on the two sides of the chisel edge. Every primary relief surface includes a cutting edge, a knife-back edge and an outer edge. Two helical cutting edges are respectively formed spirally on the outer surface of the flute part from the outer edges to form two helical grooves. Two assist relief surfaces are respectively formed on the inner walls of the two helical grooves and an included angle is between the assist relief surface and the horizontal plane. Every assist relief surface is connected with the cutting edge of one primary relief surface and portion of the knife-back edge of another primary relief surface. This drill structure can reduce the thickness of the chisel edge to decrease the resistance of the drilling and increase the life of the flute part.