The mechanical performance of semicrystalline polymers is strongly dependent on their underlying microstructure, consisting of crystallographic lamellae and amorphous layers. In line with that, semicrystalline polymers have previously been modeled as two and three-phase composites, consisting of a crystalline and an amorphous phase and, in case of the three-phase composite, a rigid-amorphous phase between the other two, having a somewhat ordered structure and a constant thickness. In this work, the ability of two-phase and three-phase composite models to predict the elastic modulus of semicrystalline polymers is investigated. The three-phase model incorporates an internal length scale through crystalline lamellar and interphase thicknesses, whereas no length scales are included in the two-phase model. Using linear elastic behavior for the constituent phases, a closed form solution for the average stiffness of the inclusion is obtained. A hybrid inclusion interaction model has been used to compute the effective elastic properties of polyethylene. The model results are compared with experimental data to assess the capabilities of the two- or three-phase composite inclusion model. (C) 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 2173-2184, 2010