Reserch to elucidate the characteristics of force-fitted cylinders and gears has been carried out for several years. However, the change in the fitting surface conditions caused by force-fit has not been considered and the change in interference has been ignored. Under such a present day situation, taking up the research of fitting surface conditions caused by force-fit as the subject of the present paper, surface roughness, wear and plastic deformation of force-fitted surfaces are examined, and the actual interference during force-fitted is discussed. From this study, it is found that the decrease in interference caused by force-fit is so large as to be impossible to ignore and the interference during force-fitting changes along the axial direction. Moreover, the experimental formula, by which the actual interference of the force-fitted gear can be calculated, is introduced.