Abstract: The frequency sequence detection and error-correcting decoding are analyzed in Differential Frequency Hopping (DFH) communication system. From the viewpoint of the third party of communications, the receiving and decoding process and the complexity of transition function analysis are discussed. The results show that DFH is not a LPI communication manner, and its security relies on the transition function. The complexity of transition function analysis is very high, which provides very high security to DFH communication system.