In this paper, the effects of radio frequency identification (RFID) reader interference are investigated in terms of the interrogation range. In order to evaluate RFID interference quantitatively, the signal-to-interference ratio (SIR) equation is initially derived, and the interrogation-reduction range ratio (IRRR) defined. IRRR is a function of the distance between a desired reader and an interfering reader. Co-channel interference (CCI) and adjacent-channel interference (ACI) instances of IRRR are simulated. Simulation results show that reader-reader distances achieving 0 % IRRR, indicating no interference between the two readers, are 1200 m and 35 m for the CCI and ACI cases, respectively. The IRRR factor is inversely proportional to the reader-reader distance in both cases. The simulation results were also verified by measurement results using an ETRI UHF RFID system. Measurement results were found to be in good agreement with the simulation results. It can be concluded that the present simulation results are reliable and applicable in analyses of more complex interfering problems in actual RFID system deployment instances.