Abstract: The surface roughness and the gaseous rarefaction effects must be taken into account in the case when the thickness of air bearing film in modern hard disk drives (HDDs) is close to or less than the molecular-mean free path. Considering gaseous rarefaction and surface roughness effects by including the pressure flow and shear flow factors, a generalized non-dimensional Reynolds equation is solved by using a least square finite difference (LSFD) method. Effects of the surface roughness modes on the load carrying characteristics of air bearing films in HDDs with ultra-low flying heights (1-2 nanometres) are investigated. Numerical results show that the effects of the surface roughness modes on the pressure distributions and the load carrying capacity are obvious, while those on the pressure center are weak. It will increase the pressure distribution and the load carry capacity of the air bearing films for the head with transverse roughness among various surface roughness modes.