A new experimental method for determining the dependence of the electron mobility on the longitudinal-electric field has been developed. The development, validation and explanation of this new method has been carefully carried out. We have applied this procedure to standard submicron MOSFETs and after having obtained the mobility dependence on both the transverse- and longitudinal-electric fields we reproduced the experimental output curves. The saturation velocity has also been calculated using the mobility curves obtained by this new method.