Many of modem radars use modulated pulse repetition intervals for the purpose of anti-aliasing and ECCM. The interception, analysis and identification of radar signals is a major function of a radar intercept receiver. In this paper, we discuss the identification of pulse repetition intervals modulation of radar signals which is one of the major parameters for the analysis of radar. We proposed a new algorithm based on Markov models approach. This approach is shown to be reliable and robust to the missing pulses, as well as to require only relatively few pulse data.