Summary form only given. Lubricant flow on a hard-disk due to a flying head slider was studied using an optical surface analyzer. A cross section of the lubricant surface under a nano slider is shown. We found that the "lubricant depletion depth" due to air bearing pressure was inversely proportional to the disk rotational speed because of the restoring effect caused by the centrifugal force and air shear force, and with some effect of the surface topography. In addition we analyzed the lubricant thickness modulation in the circumferential direction. It was found that a lubricant pooling area existed on the depletion tracks in some places but no strong natural frequency signal was detected in this case.