Nitrogen adsorption at 77 K on metal-organic framework (MOF) is investigated by means of molecular simulations. We consider both regular Cu-BTC crystal and a MOF-based hierarchical porous solid consisting of a mesopore carved out of a Cu-BTC crystal. The t-plot method is applied to these solids by using a non-porous Cu-BTC surface as the reference sample. The values of the mesoporous and external surface areas are determined from the t-plot, and the validity of the method for this type of hierarchical solid is discussed.