Factors related to interlocking, which is known to affect the core magnetic properties of motors and generators, were analyzed with the aim of achieving greater accuracy in the prediction of core losses. In a ring core sample assembled by V-type interlocking, dowel formation and dowel jointing showed comparable contributions to iron loss increase at low frequencies (e.g., 50Hz), whereas at high frequencies, increases in iron loss due to dowel jointing were greater than those due to dowel formation. It was suggested that dowel formation increased iron loss because of the strains originating from the plastic constraint due to the existence of dowels. Jointing of individual dowels increased eddy current loss at higher frequencies (e.g., 400Hz). Furthermore, eddy current loss increased significantly under the existence of an interlinkage magnetic flux across the line between two adjacent dowels, including the case of staggered dowel arrangements. The increase in iron loss in a motor core owing to interlocking can be estimated by the obtained degradation tendencies of iron losses in ring cores with respect to the density of interlocking dowels.