The effective Young's modulus of starch-zein biopolymer composite is studied a finite element model. This model handles the fact that starch-zein interface is not perfect in the sense that elasticity properties across the interface are altered. The motivation here is to predict the right modulus of elasticity near the interface for any zein content. In that way, the finite element model requires zein, starch and interphase properties to be implemented for the computation of the composite Young's modulus as a function of zein content. The main variables are the interphase thickness and modulus. The comparison between the predicted and experimental results shows, firstly, that the effective composite properties are not correctly estimated using standard models with perfect interface hypothesis. Secondly, the three-phase model is able to suggest optimal interphase parameters in order to fit the experimental data obtained using three-point bending test. Finally, the optimal interphase parameters (thickness and modulus) vary as a function of zein content.