Based on the theory of passive RFID technology and antenna scattering, some computation methods for radar cross-section of tag antennas with different loads are derived, and by integrating the radar cross-section with electromagnetic propagation environment, the identification range of passive radio frequency identification is theoretically analyzed. Furthermore, by measuring backscattered power in the case of different loads, and then combined with the calculating methods, radar cross-section of the tag is also found. Theoretical analysis results are found to be in good agreement with experimental data. The results obtained are useful for optimizing tag performance in passive RFID systems.