The lubrication mechanism of concentrated polymer brushes (CPBs) exhibiting ultralow frictional property was investigated. The frictional force and hence the frictional coefficient mu between CPBs of polystyrene (PS) were measured as a function of shear velocity v and degree of swelling. The degree of swelling was precisely controlled by varying the composition of solvent, which consisted of a mixture of toluene (good solvent for PS) and 2-propanol (nonsolvent for PS), from the brush highly stretched state (toluene rich) to glassy state (2-propanol rich). The mu data of the mixtures revealed two lubrication mechanisms, i.e., boundary and hydrodynamic lubrication. Boundary lubrication with mu values less dependent on shear velocity was observed both in ultralow (mu on the order of 10(-4)) and high frictional (mu on the order of 0.1) regimes. On the other hand, hydrodynamic lubrication was well described by the relation mu = beta center dot v(alpha) with a having an almost constant value of ca. 0.7. It was found that parameter beta depended on the solvent composition and was scaled by the degree of swelling. It should be noted that the confronted polymer brushes interacted with each other even in this regime. Thus, CPBs in solvents may be employed as an efficient lubricating layer due to their unique features.