Ultrasonic　surface　rolling　(USR)　treatment　was　subjected　to　the　25CrNi2MoV　shaft　steel　surface.　The　rolling　contact　fatigue　(RCF)　properties　of　treated　samples　under　different　lubricant　viscosities　were　investigated.　Results　showed　that　the　USR　treatment　significantly　refined　the　surface　grains,　improved　the　surface　hardness　and　generated　a　compressive　residual　stress　(CRS)　field　in　the　subsurface　layer.　Reduced　surface　roughness　changed　the　lubrication　regime　between　contact　pairs　and　relieved　the　local　surface　stress　concentration.　USR　treatment　is　an　effective　way　to　improve　the　RCF　life　under　high　lubricant　viscosity.　Moreover,　the　dominant　failure　mode　of　USR　treated　sample　was　delamination.