Copyright 2015 Society of Petroleum Engineers. Organic rich shale contains kerogen nanopores with characteristic lengths typically less than 100 nm. Adsorption behavior of hydrocarbon gas in the nanopores and the adsorption amount need to be considered for accurately calculating gas-in-place of this type of reservoir. We investigated CH4 behavior in kerogen nanopores under 350 and 400 K. A slit-like pore is constructed with full-atom kerogen models. The corresponding aromaticity is 0.71 and the structure is similar to coal. Two different cases are considered: Pressure and temperature are fixed with isobaric-isothermal ensembles, and different pore sizes from 2 nm to 40 nm (as mesopore models) are obtained by changing the number of CH4 molecules (Case 1); pore size and temperature are fixed with canonical ensembles, in which pressures are ranged from 5 MPa to 70 MPa by changing the number of CH4 molecules (Case 2). For case 2, four different pore sizes from 7.5 nm to 20 nm are useed. It was found that the density of the bulk region in an approximately 40 nm pore was almost the same as that of the bulk phase at the same pressure. However, it becomes higher as the pore size becomes smaller. Because the kerogen molecule is large in terms of size, the packing of kerogen molecules always leaves some cavities, which are capable of hosting CH4 molecules. This is similar to the micropores in activated carbons. The mono-molecular layer model, like the Langmuir model, may not be appropriate. On the other hand, the extended Dubinin-Radushkevich (DR) equation is ready to be implemented to acquire adsorption isotherms for those micropores. The DR model was developed to describe pore-filling state of adsorption behavior in micropores, which has been extended to supercritical conditions by assuming saturated vapor pressures. As anticipated, it was found that the adsorption isotherms were described by the extended DR equation better than the Langmuir model for all pore-size systems that we have studied. Moreover, we show that it is possible to apply the DR equation to describe the gas status in the bulk region inside the mesopores. Our work offers a better understanding in the calculation of gas-in-place in organic rich shales providing that we have the pore size distribution and constituents of a certain kerogen.