Abstract: Disclosed are in-vivo interstitial antennas (IVIAs) for thermal treatment and deactivation of tumors by means of microwaves. An IVIA comprises a microwave monopole antenna (MMA) and a medical catheter, and the MMA is inserted into the medical catheter to form the IVIA. The MMA comprises coaxial cable and three types of capacitors. The coaxial cable consists of first and second conductors and a first insulator, and only the first conductor extends less than a quarter wavelength. The first capacitor is located around the end of the extended first conductor and includes the second insulator and the third conductor. The second and third capacitors are located between the first capacitor and the apertures of the MMAs and have about same function. Because of arbitrarily changed input impedance of the first capacitor, almost perfect matching can be achieved and desirable temperature distributions can be obtained due to the second and third capacitors.