Development of high-performance polymer composites exposed to water lubrication conditions is of increasing interests for numerous applications, where high durability and reliability are demanded. However, formation of a protective boundary reaction layer on the rubbing surfaces can be problematic with the presence of water. The roles of hexagonal boron nitride (h-BN) nanoparticles on the tribological performance of polyoxymethylene (POM) and POM composite reinforced with short carbon fibers were investigated. It is identified that the addition of low loading h-BN greatly improves the tribological performance, e.g. wear resistance of POM is enhanced by one order of magnitude. Moreover, h-BN and carbon fibers play a synergetic role in enhancing the wear resistance. Tribo-chemistry and nanostructures of the boundary film were comprehensively investigated. It is revealed that H₃BO₃ and B₂O₃ generated as products of tribo-chemical reactions are arrayed in a closely packed outmost layer of the boundary film and exert an important influence on the tribological performance. Our work gives the evidence that the basal planes of H₃BO₃ and B₂O₃ are aligned parallel to the sliding direction, leading to low friction and wear.