Hyperthermia can enhance the radiation effect as a synergistic effect in combination of X-ray irradiation and hyperthermia. Hyperthermia sensitizes radioresistant S-Phase cells in cell cycle and inhibits cellular recovery from sublethal damage. Author fabricated 100 watt, 2450 MHz microwave applicator for hyperthermia, planned the methods and conditions of heating, and measured the temperature by using Agar phantom as a preliminary test. For biological examination, 90 rats were divided into 4 groups as hyperthermia, X-ray irradiation(6, 8 and 10 Gy), X-ray irradiation with hyperthermia, and normal control groups. Histologic examination of the liver was done. The results were as followings: 1) In hyperthermia group, there was mild degeneration of hepatocyte and mild degree of edema in portal tracts. 2) Mild degeneration of hepatocyte was appeared in the liver after 15 days of 6 Gy X-ray irradiation, and mild necrosis of hepatocyte with disarray after 60 days of 8 Gy irradiation. the findings were more severe in the group of 10 Gy irradiation. 3) In the group combined modality of irradiation plus hyperthermia, mild degneration of hepatcoyte was appeared after 15 days of 6 Gy irradiation, and mild necrosis of hepatocyte with disarray after 60 days of 8 Gy irradition. 4) The thermal enhancement ratio (TER) of the liver, which was calculated at the end point of disarray and necrosis of hepatocyte, was 1.0 in the group of combined modality.