This paper describes a study of the interbar current losses in induction motors under no-load condition by comparing the measured and calculated results. The interbar resistances and losses are measured on the machine of the rotor with end rings removed. The increase in interbar resistances caused by the slot insulation treatment is ascertained to decrease the losses effectively. The losses are calculated with three-dimensional (3-D) magnetic field analysis on finite-element model reflecting the measured interbar resistance. The calculated losses agree well with the dependence of the measured losses on the interbar resistances. Moreover, it is confirmed that the core heating treatment caused the loss reduction and the increase of the high-frequency impedances form measurement results.