Low friction mechanism of tribo-coating from view point of structure of tribo-coating film was studied. Solid lubrication film formed by tribo-coating, which deposits a solid lubricant by evaporation to the contact interface during friction in vacuum, gave low friction coefficient below 0.03 that can not be observed by any other solid lubricants of soft metals. The tribo-coating film formed on the pin had nano-order composite structure, which the crystalline indium of nano size were distributed in an amorphous matrix of silicon oxide and chromium oxide. Because of the nano composite structure, a very thin indium film was formed without break down like conventional pre-coated thin film. The thinner indium film could give smaller value of friction coefficient than that of conventional solid lubricant. This is an abstract of a paper presented at the World Tribology Congress III (Washington, DC 9/12-16/2005).