A theoretical methodology is developed to study the permeability of gas in organic tight porous media. In derivation of our equations, three main mechanisms of gas transport in organic porous media are taken into account. The modified fractal theory is used to explain viscous transport. Using the kinetic theory of gases, a similar formula derived in our previous study (Behrang et al., 2016) is applied to study the slippage phenomenon and the Knudsen transport. The surface transport which shows the impact of the gas adsorption on the permeability is addressed by direct solution of the Boltzmann transport equation for a thin layer of adsorbed gas. The final equation is used to explore influences of the adsorbed layer thickness, grain surface specularity and pressure on the gas permeability. The presented approach is validated against available experimental data. An excellent agreement between our proposed theoretical model and experimental results are observed.