A unit-cell of Frequency Selective Surface (FSS) which allows for a concentration of the electromagnetic (EM) energy on a deep-seated tissue in the human body is proposed. The proposed FSS unit-cell can change the phase response of the EM wave in a wide phase range, while maintaining the minimum variation of the transmission magnitude within 1 dB. By integrating the unit-cell into the phased-array configuration, the EM energy can be localized into the deep-seated tissue by compensating the phase-delays for different electrical lengths between the FSS-array and the desired target. It leads to the constructive interference of the EM field at the target tissue, so as to deliver strong energy into the human body. The unit-cell consisting of the three-layer metallization is embedded in the water-bolus which provides the optimal condition for the wave-penetration into the complex dielectric media. The FSS unit-cell is analyzed by using the equivalent circuit model and the transmission characteristic of the EM wave passing through the FSS is evaluated on the phantom as an equivalent model for human body. The proposed unit-cell is designed at 915 MHz allocated in Industrial Scientific Medical frequency band.