Purpose : To determine the effects of power and coagulation time on lesion size of ex-vivo bovine liver usingmicrowaves. Materials and Methods : Six bovine livers were divided into two groups(first group: 30W output, secondgroup: 60W output) and microwave coagulation was performed for 30, 60, and 120 sec. thermal injury site was thenobserved by means of sonography, and the maximal transverse diameter of the echo-change portion after microwavecoagulation was measured. On the section of specimen, maximal transverse diameters of the thermal injury site weremeasured by gross inspection and compared with the result of sonographic measurement. Results : Maximal transversediameters of hyperechoic lesions of the first group, as seen on sonography were 8.3 mm, 12.2mm, and 15.6mm, andthe maximal transverse diameters of thermal injury sites on gross specimens were 9.1mm, 12.0mm, and 15.1mm,respectively. Maximal transverse diameters of hyperechoic lesions of the second group, as seen on sonography, were12.1 mm, 17.4 mm, and 21.2 mm and maximal transverse diameters of thermal injury sites on gross specimens were13.2 mm, 16.0 mm, and 20.0 mm, respectively. Statistically maximal transverse diameters of hyperechoic lesions, asseen on sonography, correlated closely with the gross findings of maximal transverse diameters of thermal injurysites(P<0.05). Conclusion : Maximal transverse diameters of thermal injury sites were significantly increased asthe output of the microwave coagulator and the duration of coagulation time increased(P<0.05).