A new calibration method of frictional forces in atomic force microscopy (AFM) is suggested. An angle conversion factor is defined using the relationship between torsional angle and frictional signal. When the factor is measured, the slopes of the torsional angle and the frictional signal as a function of the normal force are used to eliminate the effect of the adhesive force. Moment balance equations on the flat surface and the top edge of a commercial step grating are used to obtain the angle conversion factor. After the factor is obtained from an AFM system, it can be applied to all cantilevers without additional experiments.