Abstract: The effects of instability in repetitive signals acquired from rotatingmachines, can be critical in the extraction of Periodic signatures by cepstrum analysis for diagnostic applications. In this Paper, the rePetit-ive signal is decomposed into two components, i. e. waveform an'd rePeti-tive Pulse series. The effects of instability are represented by the levels of magnitude modulation, phase modulation, wave deformation and background noise. Based on the theory of deconvolution, a discussion on Perio dic signature analysis through cePstrum is Presented, which shows that Periodic signatures can be represented by the harmonics in cepstrum which only have relationship with the level of instability. This results can be very helpful for practical applications.