The previous knowledge of the hobbing forces is useful to the design of hob tooth strength, the decision of optimum cutting conditions and the analysis of vibration on the bobbing machine. The cutting forces are calculated from the cross sectional area of uncut chip in hohbing and the specific cutting force of gear material. In the calculation of hobbing forces, the effect of the following factors on the cutting forces is investigated. The factors are (1) the direction of chip's flow on rake face of hob tooth, (2) the difference of specific cutting forces by continuous and intermittent turning tests, and (3) the frictional angle in hobbing. The tangential forces are almost the same in both calculated and measured forces, but the thrust forces are different at changing point of direction of the force. Movement of the table of hobbing machine caused by backlash effects the measured thrust and axial forces.