Abstract: The current thickness of the gas film in air bearing of modern hard disk drives is only one order of magnitude larger than the diameter of gas molecules. At such a small thickness, the nanoscale effect cannot be neglected. Therefore, it is essential to understand the behavior of the ultrathin gas film lubrication. Thus a nanoscale effect function, N_p, is proposed, with the unidirectional flow of the rarefied gas between two parallel plates to be investigated based on the kinetic theory and the gas behavior in the near boundary regime to be focused on. The function N_p is dependent on the inverse Knudsen number, D, which characterizes the gas rarefaction. The FK-Boltzmann lubrication model in current use is therefore modified by taking into account N_p. And the numerical calculations with respect to a rarefied gas system confined between two infinite flat plates are carried out based on the modified Reynolds equations. It has been found that the nanoscale effect is minute and can be ignored when the inverse Knudsen number is larger than 1.772. However, when the inverse Knusden number is smaller than 0.886,the nanoscale effect is prominent, which leads to a significant reduce of the flow rate and slight increase of the pressure gradient and load capacity. This indicates that the nanoscale effect plays an important role in ultrathin gas film lubrication.