Abstract : In this paper, serial inventory systems with Poisson external demand and fixed costs at each stock are considered and the guaranteed-service approach (GSA) is used to design their optimal echelon (R, nQ) inventory policies. The problem is decomposed into two independent sub-problems, batch size decision sub-problem and reorder point decision sub-problem, which are solved optimally by using a dynamic programming algorithm and a branch and bound algorithm, respectively. Numerical experiments on randomly generated instances demonstrate the efficiency of these algorithms.