In this report, we adopt metal cores made of amorphous metal (2605SA1, Metglas, Inc) to minimize the iron loss in electrical motors, and compare the efficiency improvement with that obtained with electrical steel cores. First, we measured the iron loss of the cores by the improved exciting current method and the H coil method and found that the iron loss of the amorphous metal core is about one-fifth that of the electrical steel core. Then we built 11kW output motors with each type of core and compared the efficiencies. The efficiency improvement with the amorphous metal cores is 1.5% at the rated power and speed.