This paper describes a method of determination of the optimal sizes and locations of cylindrical air gaps inside the multi-slot coaxial antenna operating at 2.45 GHz. The dimensions of the microwave antenna with a 50-ohm coaxial feed were revised with regard to antenna reflection coefficient distributions. It should be emphasized that there are some differences between the operating frequencies of such antennas described in the literature and the resonant frequencies obtained from the antenna characteristics. It may significantly affect the interstitial hyperthermia treatment. It is also possible to resize the antenna slots to minimalize the S11-parameter value and obtain the best antenna impedance matching inside various tissues commonly treated in this method. The numerical implementation of such multi-physical problem is based on the finite element method.