The primary friction mechanism between boundary-lubricated sliding surfaces was investigated. Experiments were performed on well-polished aluminum, copper, and chromium using mineral oil lubricant. It was found that the prevailing boundary lubrication model, which is based on the adhesion between asperities and shearing of the lubricant film, cannot account for the formation of plowing grooves on polished surfaces. Scanning electron micrographs of the worn surfaces and surface profiles have shown that plowing is the dominant mechanism of friction in boundary lubrication. Theoretical analysis has shown that the coefficient of friction depends on the sharpness and the size of the entrapped wear debris or the surface asperities, and the interfacial “frictional” conditions. Reasonable agreement was obtained between theoretical and experimental friction coefficients.