Even though semi-crystalline polymers are closer to composite, even nano-composite material, little work has been done to predict their properties as in the case of composite or filled polymer. In their work Halpin and Kardos  proposed to determine the elastic modulus of semi-crystalline polymers. The lamellae are supposed to be like fibers. An adjustable parameter in this model was linked to the crystallite shape ratio. However, this model is well adapted for low volume fraction, which is not the case of semi-crystalline materials like Polypropylene (PP) and Polyethylene (PE). Other authors were interested in the evolution of the microstructure and large deformation of the semi-crystalline polymer using a new developed model . The main ideas of the model were adapted by other authors to predict the elasto-visco-plastic behavior [3,4]. The predicted initial modulus variation of PE versus the crystalline fraction does not fit the experimental values. The aim of our work is to establish a relationship between the microstructure and elastic properties of the semicrystalline polymer. To fulfill this goal, experimental investigations of two kinds of polymers, Polypropylene and Polyethylene, are done. Micro-mechanical models are tested in order to choose which model is better adapted for these materials. A comparison of modeling and experiments will be presented.