The methodology of predicting radio frequency identification (RFID) passive tag power supply system reliability is developed. The reliability of a RFID passive tag power supply system is estimated in terms of a parametric failure associated with the I-V characteristics drift of resonant-tunneling diodes (RTD) that are a part of the tag rectifier under the influence of external factors beyond the tolerance limits. The methodology of the time to failure of the RFID passive tag power supply system calculation by the criterion of the minimum permissible range is presented. The developed methodology can be used in the design of RFID systems with passive UHF and SHF tags to predict their reliability under specified operating conditions. The developed algorithms and software modules can be integrated into the corresponding CAD systems. © 2018 IEEE.