Objective:To investigate the changes of actual power of percutaneous microwave coagulation and the size of coagulation area under different operating time and power through performing coagulation on pork liver in vitro using water refrigeration cycle microwave antenna. Methods:Thirty fresh pork livers underwent thermal ablation using MTC-3 water refrigeration cycle microwave antenna with different operating times and powers,respectively. The change of powers of thermal ablation and the size of the coagulation area were measured. Results:The deviation between the actual power and objective power was increased with the increase of objective power. While the deviation between the actual power and objective power showed no relation with the operating time. The size of coagulation area was increased with the increase of operating time and objective power. It was more likely that the coagulation area looked like a ball when the objective power was 60-70 W and the operating time was 15 min. Conclusion:The actual power of thermal ablation is important to the efficacy. With the increase of objective power,the attenuation of power is increased. The coagulation area shows as a ball with an objective power of 60-70 W and an operating time of 15 min. This study might provide evidence for thermal ablation used to treat liver cancer.