The heating characteristics of an RF rectangular cavity applicator for hyperthermic treatment that targets deep-seated tumors were investigated numerically and experimentally. In the numerical study, Maxwell's equations and heat transfer equations were solved for a dielectric phantom with and without blood flow. Conductive caps attached to the dielectric phantom to shield the non-tumor regions. The experimental study showed the validity and possibility of heating deep-seated tumors. Thus, the rectangular resonant cavity applicator with an L-type antenna can heat deep-seated tumors.