In relation to the machinability of drilling operation, experiments were made to investigate the effect of cutting condition on static as well as dynamic cutting resistances in cutting plane carbon steel (SM 45 C) with H.S.S. twist drills. The results were as follows. 1) The static cutting resistances on carbon steel can practically be calculated by the following equations which were derived from experimental result. The deviation from the experimental values was less than 8% and 13% for cutting torque and thrust respectively. For cutting torque M: M=0.019 $H_B;{f^{0.68}d^{1.68}$ For thrust T: T=0.400406 $r^{0.6}d^{0.68}$ + 0.1835 $H_BC^2$(where $H_B$: Brinnel hardness) 2) The static components of cutting resistance are increased exponentially with increasing drill diameter and feed rate. On the effect of drill diameter, the dynamic components of torque are decreased with increasing dirll diameter because of rigidity, the dynamic components of thrust being not effected with the changes. 3) As feed rates increase, the dynamic components of torque rather decrease although its changes on thrust components are unstable. 4) The static components of cutting resistance and dynamic component of torque are slightly decreased in accordance with the increase of spindle speed although its dynamic thrust components are not effected by the spindle speed.