In this research, stiffness of polymer-clay nanocomposites was simulated by Mori-Tanaka and two and three dimensional finite element models. Nanoclays were dispersed into polymer matrix in two ways, namely parallel and random orientations toward loading direction. Effects of microstructural parameters including volume fraction of nanoclays, elastic modulus of nanoclays and interphase, thickness of interphase, aspect ratio of nanoclays and random orientation of nanoclays on elastic modulus of the nanocomposite were investigated by finite element model. Comparing the simulation with experimental results showed that the Mori-Tanak simulation results were closer to the experimental results. Analysis of results showed that the volume fraction of nanoclay, elastic modulus of nanoclay, deviation of nanoclay layers with respect to loading direction, nanoclays aspect ratio, thickness of interphase and the elastic modulus of interphase had respectively the most to the least effect on elastic modulus of nanocomposite.