Stagger Pulse Repetition Interval (PRI) modulation has been widely used in radar systems. Stagger PRI modulation is hard to recognize because it has the same characteristics with jitter PRI modulation. Electronic support systems have to work with spurious and missing pulses due to hard environment conditions. This situation also makes it difficult to recognize the stagger PRI modulated signals. On the other hand, since response time of the system is also very important for the electronic support systems, the designed method should be simple and causal. In this paper we propose a new causal recognition method ,that does not use histogram based techniques, for stagger PRI modulation in dense environment. © 2012 IEEE.