The aim of this article is to try to explain why isotactic polypropylene (PP) is stiffer than high density polyethylene (HDPE) despite the fact that this latter is more crystalline and that its crystallites are stiffer than PP ones. Two micromechanical models were chosen for their ability to represent semi-crystal line polymers. The first one is a differential scheme in which ellipsoidal crystallites are randomly dispersed in an amorphous matrix. The second one is a self-consistent scheme where the material is considered as an aggregate of randomly oriented two layered-phase composite inclusions (crystalline-amorphous). Experiment-model comparisons are clearly in favor of the first model. This latter demonstrates the key importance of the crystalline lamellae aspect ratio on the elastic properties of semi-crystalline polymers. (C) 2004 Elsevier Ltd. All rights reserved.