The formation of graphene layers on MgO-supported Pt nanoparticles was studied by both in situ and ex situ high-resolution transmission electron microscopy (HRTEM). The HRTEM images indicate that graphene sheets grow from steps in the surface of Pt nanoparticles. The subsequent morphology of the graphene sheets is a strong function of Pt particle size. For particles less than similar to 6 nm in diameter, the graphene sheets form nanotubes or move from the surface of Pt particles and accumulate on the MgO support. Complete particle envelopment by multiple graphene layers was only observed for particle greater than similar to 6 nm in diameter. The observed dependence of graphene morphology on Pt nanoparticle size and shape is associated with the strain energy generated between graphene layers during their growth and the overall free energy of the graphene-Pt system. (C) 2011 Elsevier Inc. All rights reserved.