We modeled condensation phenomena within cylindrical nanopores as a possible replacement for the Kelvin model that underestimates nanometer order pore sizes. The proposed model follows the simple concept of a continuum assumption similar to that for the Kelvin model. The difference was in the introduction of the contribution of the pore-wall potential and the curvature-dependent surface tension in our model. A molecular dynamics (MD) technique developed by the authors for isotherm determination was employed to test the concept and the model. Several isotherms for Nn-like Lennard-Jones (LJ) particles in a silicate-like cylindrical pore with various diameters from 2 to 4 nm were obtained through MD simulations, and a relation between pore diameter and critical condensation pressure was determined. The present model successfully described the relation to demonstrate its reliability. The validity of the proposed model was examined also from the aspect of the shape of the meniscus and the pressure profile in the condensed phase, and gave fairly good agreement.