<p>Couplings are mechanical components that transmit the drive torque between two rotating parts. The characteristics of entire mechatronic systems are influenced by the characteristics of couplings. Therefore, predicting the characteristics of couplings is crucial. However, as couplings have frictional contact surfaces, predicting their characteristics is difficult because of nonlinearity. This study aims to investigate the torsional stiffness of a coupling considering the stiffness of its contact part. To this end, a finite element method incorporating surface asperity deformations and accounting for contact stiffness was developed. The fixing torque and the clearance between the shaft and the borehole affect the surface pressure, whereas the contact stiffness depends on the surface properties. To compare our findings with the simulation results, stiffness measurement tests are conducted. The results confirm that the developed model can adequately predict the torsional stiffness of the coupling, which depends on the fixing torque.</p>