The operation of passive anti-collision RFID system is characterized by the interrogation zone which is estimated in any direction of 3D space for a group of electronic transponders. The elements of algorithm for determining the 3D interrogation zone in an inductively coupled anti-collision RFID system have been presented in this paper. The energy conditions have been analyzed in detail. The algorithm based on the Monte Carlo method and the computer program implemented in MathCAD engineering calculation software has been utilized in order to achieve the posed aims