In this paper, we propose a modified I/Q diversity scheme receiver of UHF RFID reader system. The modified I/Q diversity receiver is more robust than the conventional homodyne receiver in the wireless noisy, fading channel and phase noise environments by making use of additional axes. In particular, it is shown that the closer the phase difference ${ heta}(t)$ between the reader and the tag to ${pi}/4$, the larger performance improvement we can get. The performance of the proposed receiver is verified by equations and is demonstrated by the computer simulation for various difference ${ heta}(t)$ cases.