The formation mechanisms of wrinkles in single layer graphene sheets and graphene oxide (GO) papers, as well as their effects on the corresponding Young's moduli have been studied based on the molecular dynamics simulations. Wrinkling occurs in GO papers in the form of undulation as a consequence of the interactions between the adjacent individual GO sheets. In particular, the edge-to-edge interactions play a dominant role in determining the wrinkled structure, whereas the face-to-face interactions hardly cause wrinkles as they are thermodynamically stable with inherently low potential energies. Both graphene sheets and GO papers with wrinkles show two different modes of deformation in tension. Although wrinkles have only a negligible effect on Young's modulus of the individual graphene sheets, they reduce the Young's modulus of GO papers by as much as 60% of the pristine value. (C) 2013 Elsevier Ltd. All rights reserved.