In polymer nanocomposites, the interfacial region plays a key role in the reinforcement of materials properties. Traditional two-phase micromechanical models usually ignore the contribution of such interfacial region to the overall materials properties. In this study, we use molecular dynamics simulation to determine the effective size and the Young s modulus of effective clay clusters which are regarded as basic building blocks in clay-based polymer nanocomposites. Two types of clay clusters are considered: one is fully exfoliated clay and another is partially exfoliated clay. The calculated Young s modulus of effective clay clusters can be used to predict the overall mechanical properties of clay-based polymer nanocomposites.