In this paper we show a theoretical criterion of choice between a FSOS (fixed-size ordering system) and a FIOS (fixed-interval ordering system), and then propose a mixed ordering policy which is incorporated in the elements of both types of ordering systems. The mixed policy designed here combines the feed-feedback mechanism of a FIOS with the on-off (reorder point) mechanism of a FSOS. More specifically, the mixed policy orders a required quantity by taking into account the forecast demand and term-end inventory whenever the current inventory breaks a reorder point forecast at each period. We will prove the advantages of this mixed system by comparing between the performance of the two systems.