The authors have been studying thin coaxial-slot antennas for interstitial microwave thermal therapy. In this paper, firstly, the structure of the coaxial-slot antenna is briefly reviewed. Secondly, the procedure for temperature calculation around the antennas and the calculation model based on the CT images of the patient are described. Thirdly, the blood flow rate of the tissue concerned during the treatment is estimated from the temperature transition during the treatment. Finally, the temperature distribution in and around the tumor is calculated based on the estimated blood flow rate and some conditions for the actual treatment.