The focus of this study is to understand how exfoliated graphite nanoplatelets (xGnP) alter the elastic response of semi-crystalline polymers and reveal the mechanisms responsible for the property enhancement observed in graphite reinforced polymer nanocomposites. The nanocomposites were made by coating of polyamide 12 (PA12) powder with xGnP followed by melt compounding and injection molding. The mechanical properties, including tensile strength and modulus, flexural strength, and thermo-mechanical properties such as the glass transition temperature (T-g) and loss modulus of the xGnP/PA12 nanocomposites, were determined as a function of the xGnP content. It was found that xGnP acted as nucleating agent for PA12 and significantly altered the physical properties of the polymers including T-g and crystallization characteristics. Morphological studies indicated formation of xGnP agglomerates at xGnP content as low as 3wt.%. A strong correlation between the T-g and the tensile modulus in xGnPPA12 was determined revealing presence of secondary reinforcing mechanisms in xGnP reinforced semi-crystalline polymer nanocomposites.