Abstract: The lightweight cipher LBlock was proposed at ANCS in 2011. It has the structure of Feistel and is widely applied in the security of Internet of things (IoT). In this paper, a cipher-text fault analysis for LBlock cipher by injecting faults is proposed, and it is analyzed by 6 distinguishers in the last but 3 rounds. On the basis of original distinguishers as SEI, GF, GF-SEI, MLE, we propose GF-MLE and MLE-SEI distinguishers as new distinguishers. The simulation experiments show that the secret key can be recovered with over 99% success probability in a short period of time, and these two new distinguishers can not only improve the attacking efficiency, but also decrease the number of faults. This shows that the ciphertext-only fault analysis poses a great threat to the security of LBlock cipher.