In this work, we derive a modified slip model for gas lubrication based on the effective viscosity concept and a kinetic-theory-based slip-boundary condition. A more accurate expression for the effective mean-free path is proposed, which can overcome the deficiency of other slip models in predicting effective mean-free path. In comparison with the existing slip models, such as the first, second, and 1.5-order slip models, the current slip model agrees well with F-K model over the entire flow regimes and therefore is expected to be more accurate for ultra-thin gas film lubrication at head-disk interface.