A screw comprising a screw head having an engaging recess (11a, 11b, 11c) recessed in a top surface of the screw head; and a shank (20a, 20b) extending downwardly from a bottom of the screw head and having a central axis; an imaginary plane being perpendicular to the central axis; a threaded segment, a cutting segment, and a tapered segment sequentially arranged downwardly; a thread spirally surrounding an external surface of the shank (20a, 20b), extending downwardly from the threaded segment to the tapered segment of the shank (20a, 20b), and having a first lead angle (θ1) formed between the thread and the imaginary plane and being an acute angle; multiple cutting ribs arranged around the external surface of the shank (20a, 20b) on the cutting segment of the shank (20a, 20b); and multiple chip recesses formed between the cutting ribs, and each of the chip recesses formed between two of the cutting ribs being adjacent to each other; and characterized in that each of the cutting ribs has a rounded crest and a second lead angle (θ2) formed between the cutting rib and the imaginary plane and being an acute angle; wherein the second lead angle (θ2) is larger than the first lead angle (θ1); each of the chip recesses has a rounded concave root; the cutting segment of the shank (20a, 20b) has a first diameter being a diameter of an inscribed circle of the rounded concave roots of the chip recesses and a second diameter being a diameter of a circumcircle of the rounded crests of the cutting ribs; and the threaded segment of the shank (20a, 20b) has a third diameter being a minor diameter of the threaded segment, wherein the first diameter is smaller than the third diameter and the second diameter is larger than the third diameter. a screw head having an engaging recess (11a, 11b, 11c) recessed in a top surface of the screw head; and a shank (20a, 20b) extending downwardly from a bottom of the screw head and having a central axis; an imaginary plane being perpendicular to the central axis; a threaded segment, a cutting segment, and a tapered segment sequentially arranged downwardly; a thread spirally surrounding an external surface of the shank (20a, 20b), extending downwardly from the threaded segment to the tapered segment of the shank (20a, 20b), and having a first lead angle (θ1) formed between the thread and the imaginary plane and being an acute angle; multiple cutting ribs arranged around the external surface of the shank (20a, 20b) on the cutting segment of the shank (20a, 20b); and multiple chip recesses formed between the cutting ribs, and each of the chip recesses formed between two of the cutting ribs being adjacent to each other; and characterized in that each of the cutting ribs has a rounded crest and a second lead angle (θ2) formed between the cutting rib and the imaginary plane and being an acute angle; a central axis; an imaginary plane being perpendicular to the central axis; a threaded segment, a cutting segment, and a tapered segment sequentially arranged downwardly; a thread spirally surrounding an external surface of the shank (20a, 20b), extending downwardly from the threaded segment to the tapered segment of the shank (20a, 20b), and having a first lead angle (θ1) formed between the thread and the imaginary plane and being an acute angle; multiple cutting ribs arranged around the external surface of the shank (20a, 20b) on the cutting segment of the shank (20a, 20b); and multiple chip recesses formed between the cutting ribs, and each of the chip recesses formed between two of the cutting ribs being adjacent to each other; and characterized in that each of the cutting ribs has a rounded crest and a second lead angle (θ2) formed between the cutting rib and the imaginary plane and being an acute angle; wherein the second lead angle (θ2) is larger than the first lead angle (θ1); each of the chip recesses has a rounded concave root; the cutting segment of the shank (20a, 20b) has a first diameter being a diameter of an inscribed circle of the rounded concave roots of the chip recesses and a second diameter being a diameter of a circumcircle of the rounded crests of the cutting ribs; and the threaded segment of the shank (20a, 20b) has a third diameter being a minor diameter of the threaded segment, wherein the first diameter is smaller than the third diameter and the second diameter is larger than the third diameter.