Aging test was done by loading back method for 20kVA amorphous core transformers manufactured by Hyosung Industries Co. and korea Electric Power Corporation. Iron losses, copper losses and insulation oil temperatures of the transfromers was measured for all the testing period. Expected life of amorphous core transformers on the basis of the degradation of the insulators was 46 years at 100% load, and 2.4 years at 130% load. Average temperature rising of transformer oil of amorphous core transformers was higher than that of silicon steel core transformers. Hence lowering the oil temperature by optimized design is needed for improving the expected life of the amorphous transformers.