The objective of this research paper is to find Pn (N) the steady-state prohability that there are n units in the entire system of N-stations series queues in a general form. We obtain an implicit formula for Pn (N). Thea, we find an explieit formula for three different models in the heterogeneous case and deduce the homogeneous ease. Also we calculate both Ej^(n), E^j(m) the expected number of units in the system and the queue in the entire system, respectively