An efficient integer programming model of the dynamic lot-sizing problem is presented. The Hillier-Lieberman model is improved by a new model that has only half the number of functional constraints. The reduction on the functional constraints, together with the imposition of a properly proposed by Wagner and Whitin, significantly simplifies the model, and thus greatly improves the computation efficiency. For the 20-period problems, the new model requires only 1/369 the computation time of the Hillier-Lieberman model.