In this work, a method for calculating mobility of nano-scaled MOSFETs from the Boltzmann transport equation(BTE) is presented. Some approximations are assumed for the BTE in nano-scaled MOSFETs, and an improved distribution function of the carriers is obtained which is used to model the mobility of carriers. An analytical expression of carrier mobility is deduced considering the statistical distribution of carrier velocity and carrier life-span and taking into account especially the interaction between the longitudinal and the transverse fields. The reasonable agreement of the calculated results with simulation validated the new model, which is explicit, simple and physically well grounded.