Abstract: A nano thickness tester was used to investigate the film-forming characteristics of nanoscale SiO_2 hydrosol. Thus a glass plate coated with Cr layer was driven to roll against a bearing steel ball under the lubrication of the nanoscale SiO_2 hydrosol as a two-phase flow lubricant. The effects of the grit size of SiO_2 particulates, the concentration of the nanoscale SiO_2 hydrosol, and the rolling speed on the stability and thickness of the liquid lubricating film were examined. It was found that the viscosity of the dilute nanoscale SiO_2 hydrosol solutions was almost independent on the grit size of SiO_2 and the concentration of the SiO_2 hydrosol, however, the load-carrying capacity and film-forming ability of the dilute solutions were largely dependent on the grit size of SiO_2 and the concentration of the SiO_2 hydrosol. The stability of the lubricating film decreased with increasing grit size of SiO_2 and the concentration of the SiO_2 hydrosol, which was attributed to the damage of the SiO_2 solid particulates entrapped in the contact zone of the frictional pair to the lubricating film. Such a kind of damage of SiO_2 particulates to the lubricating film was greatly decreased with increasing thickness of the lubricating film and was almost eliminated as the lubricating film was thick enough. It was thus suggested to increase the measurement accuracy of the thickness of ultra-thin lubricating film of SiO_2 hydrosol by increasing the sliding or rolling speed of the frictional pair.