Objective:To study the feasibility and effectiveness of “water-cooled circulation” microwave coagulation(MC) in the treatment of breast tumor. Methods:The breast of pigs underwent coagulation with “water-cooled” microwave coagulation equipment-ECO100 type. After MC procedure,the volume of target area were calculated;routine pathological examination were done in the MC lesion and surrounding tissue.Adverse effects on surrounding breast tissue and skin were carefully examined during and after MC ablation. Results:Coagulation volume was postivly correlated with the power and time of MC. The correlation coefficient between MC volume and cross product of MC power with time is 0.904,regression equation is y=0.117x-2.608. Target breast cells showed coagulative-necrosis changes by HE staining after MC,no clear change were obsered in surrounding tissue by HE staining. There was not skin burn and adjacent tissue damage outside targeted tissue in the treatment of“water-cooled circulation”. Conclusion:“Water-cooled circulation” microwave coagulation could make breast cell coagulative-necrosis,and the coagulation circumscription was clear.The therapy was microinvasive,and no serious adverse effect was founded. The experiment provided a basis for the clinical application of microwave.