Capillary condensation is the vapor-to-liquid phase transition occurring in confined geometries. Such heterogeneous nucleation has been well described by the Kelvin equation, but its applicability at nanoscale is still unresolved. We show it is valid down to 0.5 nm scale when the curvature-dependent surface tension is considered. Our results that unify the validity of the Kelvin equation and the curvature effect of surface tension may provide a better understanding of nucleation phenomena.