The permeation of various pure gas (H-2, He, Ne, CH4 and Ar) through carbon membranes is investigated using a dual control volume grand canonical molecular dynamics method. A two-dimensional slit pore is employed instead of the one-dimensional pore. Compared with the experiments, simulation results show that the improvement of pore model is very necessary. The effects of membrane thickness, pore width and temperature on gas permeance and ideal separation factor are also discussed. Results show that gas permeates through membrane according to Knudsen diffusion in large pore, while Knudsen diffusion is accompanied by molecular sieving in small pore. Moreover, methane is easily adsorbed on the membrane surface due to strong attractive interactions of membrane and shows higher permeance than that of Knudsen flow. In addition, it is noted that when membrane thickness is thin enough the permeance of gas does not decrease with the increase of membrane thickness due to the strong adsorption until membrane resistance becomes dominant.