Radio frequency identification (RFID) systems need security protocols to provide confidentiality, user privacy, mutual authentication and etc. This paper analyze two of the newest RFID security protocols which proposed by Niu et al. and Fu et al. form the security viewpoint of data desynchronization attack. These two lightweight protocols were expected to proposed security protections for the RFID system and safeguard against almost all major attacks. However, we found that these two RFID security protocols were vulnerable to the attack of data desynchronization. Based on strand space model, data desynchronization attacks on these two protocols were analyzed. Furthermore, improvements to overcome the security vulnerabilities of these two protocols were given.