In some Radio-Frequency IDentification (RFID) applications, the RFID tag antennas will be printed onto flexible substrates. After sticked onto different objects, possible distortion to the tag antenna such as bending of the tag will be caused due to the flexibility. In this paper, performance degradation of passive RFID systems due to bending of the RFID tag antenna is investigated. The antenna used is the basic folded dipole. Simulation results show that the performance of a passive RFID system, characterized by operating range, can be degraded significantly due to distortion. Thus, antenna structures less sensitive to this should be considered for RFID systems where this kind of effect might occur.