Maxwell's and heat transfer equations were coupled and solved to determine the heating characteristics of an RF rectangular resonant cavity applicator for hyperthermia before the clinical stage. A simple human model with blood flow and a fat layer was constructed. The region unaffected by tumor was shielded from electromagnetic fields by using conductive caps. The surface of the human model that was exposed to the electromagnetic fields was cooled with pure water bolus. Calculated results show that this applicator can heat a deep-seated tumor.