As an ultrathin, high-flux, and energy-efficient membrane, graphene oxide (GO) has shown great potential for gas separation. Using molecular dynamics (MD) simulations, we studied the H-2/CH4 separation performance of GO membrane. It is found that the size of gas molecule and its interaction with GO sheet are two key factors in determining the diffusivity in interlayer gallery of GO, which results in significant difference of selectivity and permeability for H-2 and CH4. The influences of two interlayer configuration parameters, the interlayer spacing and oxidation degree, on the separation of H-2/CH4 are firstly studied. Then the effects of gas thermal behaviors, simulation temperature and initial gas pressure are also examined. Our studies uncover the underlying mechanism of H-2/CH4 separation in GO membrane, and these understandings could promote efficient two-dimensional layered gas separation membrane materials development by engineering assemblies. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.