A simple new method of characterization of porous materials is developed in this paper. In this method, we combine the traditional Kelvin equation with an improved statistical adsorbed film thickness to account for the finite dimension of pores. The adsorption process in a pore is viewed as a molecular layering process followed by a filling mechanism. Here, we argue that this mechanism is valid not just for mesopores but also micropores because of our allowance for the statistical film thickness to be a function of not only pressure but pore size as well. The model is tested against the data of nitrogen adsorption onto activated carbon samples: the fitting is found to be excellent, and the results are comparable with those obtained by other methods such as the DFT method. The much shorter computation time required by this method compared to that by DFT makes this method a very attractive alternative in the PSD characterization.