The molecular simulations have been performed on the permeation of pure and mixed-gases across carbon membranes with three different pore shapes: the diamond path (DP), zigzag path (ZP) and straight path (SP), composed of micro-graphite crystallites. The smallest pore width of the membranes was set at 0.5 nm and the methane and ethane, whose molecular size in the shortest diameter is approximately 0.37-0.38 nm, were chosen as permeating gases. The density profiles of methane and ethane showed that the permeation resistance inside the DP and ZP membranes was significant while the resistance of the SP membrane was localized at the membrane exit. In the case of mixed-gas permeation, the permselectivities of ethane to methane were larger than the ideal separation factors for the three membranes due to the competitive adsorption of ethane which was a strongly adsorbed component. However, the permselectivities were smaller than the adsorption equilibrium separation factors since the higher permeation resistance of ethane played a role of counteraction. It is therefore suggested that two factors, selective adsorptions and permeation resistances, should be considered for a rational design of membranes suitable for the separation of specified species in gas mixtures.