Abstract: Molecular dynamics simulations of polyethylene(PE)/graphite-nanoplatelet(GNP) composites has been implemented to investigate the morphological structures, mechanical and gas transport properties by calculation of dynamical energy, atomic radial distribution function, XRD, elastic tensor and gas diffusion coefficient, analyzing their relationship with nanofiller concentration and simulation temperature. The results indicate that GNP/PE composites exhibit in two-dimensional anisotropic structures with the graphite-nanoplatelets orientating in parallel to each other and compositing with PE matrix by evident Van der Waals force and CH-π bonding on nanoplatelet surface causing a few ordered atomic layers of PE at interface, while the PE matrix is in isotropic amorphous morphology. The nanocomposite effect of nanoplatelet with matrix leads to explicitly lower energy and results in higher Young's modulus and lower Poisson's ratio compared with pure polyethylene system. Due to the parallel orientation of GNP, the improved mechanical property represents as two-dimensional anisotropic elastic constant tensor with remarkably larger Young's modulus in orientated plane increasing with reduced temperature and incremental nanofiller concentration. Gas transports in GNP/PE composites are significantly affected by the gas barrier and orientation of GNP without gas selectivity for percolation. The graphite nanocomposites present two-dimensional anisotropic permeation of N₂, O₂ and CO₂ molecules that the diffusion coefficients are 5-8 times higher along orientation plane than in perpendicular direction of GNP when the composite nanofiller concentration increases, reasonably suggesting the material application to gas barrier and percolation control.