Molybdenum surface alloying layer was prepared on Ti6Al4V substrate using double glow discharge technique. The tribological behavior of the resulting surfacemodified layer under dry and oillubricated sliding against SAE52100 steel was evaluated on a ballondisc test rig. The results showed that the surfacemodified alloying layer was composed of Modeposited layer of 10 μm thick and Modefused layer of 15 μm thick. The hardness and elastic modulus of the surfacemodified layer were 10.859 GPa and 278.839 GPa, which were much larger than that of the Ti6Al4V substrate. The Ti6Al4V substrate was characterized by adhesion wear and scuffing under dry and lubricated sliding against the steel, while the surfacemodified layer experienced much abated adhesion wear and scuffing under the same testing conditions. This could be attributed to the different hardness and elastic modulus, and the different microstructures of the Ti6Al4V substrate and its modified counterpart. The surfacemodified layer showed poor frictionreducing and fair antiwear ability in dry sliding against the steel, while it had much better frictionreducing and antiwear ability in oillubricated sliding against the steel.