Molecular dynamics (MD) simulations were employed to investigate water-ethanol separation through mono layer graphene oxide (GO) membranes with different pore sizes and O/C ratios. The ultrahigh water flux and infinite water separation factors were achieved. The separation properties under 50/50 w/w water-ethanol mixtures reveal that higher O/C ratios favor water selectivity, and water flux is enhanced with O/C ratio and pore size. With the help of highest oxidization, water largely sorption amount governs permeation process. On the contrary, with low oxidization degree of GO membrane, water diffusion effect becomes dominant factor of permeation. We screen an optimal microstructure of GO membrane which equipped with an adequately sized pore (D = 2.4 angstrom A) and a highest O/C ratio (R = 0.49). This optimal one D2.4 angstrom_R0.49 achieves the highest water flux and fully ethanol rejection in the mimicked experimental system. This simulation study elucidates the role of O/C ratio and pore diameter in water-ethanol separation through porous GO membranes on the microscopic level and uncovers the governing effects for water permeation and also suggests a potential candidate as a water-ethanol separation membrane.