It will be noticed that the suction of air occurs to the rubbing portion of the lip of an oil seal, which is operating under sealing condition. For the purpose of studying sealing phenomena of an oil seal in this paper, the suction volume per an unit time,[Qbar]of lubricating oil instead of air is used as a means of estimating the sealing status. It becomes clear that there is the relation,[Qbar]αG1/2, whereinGis nondimensional parameter of oil seal, and that suction coefficient of oil,K (=[Qbar]/uD2G1/2, whereinuandDare sliding velocity of between shaft and seal and shaft outer diameter, respectively, depends on the sealing characteristic constant,Φ, the surface roughness of the rotating shaft and its eccentricity.Presented as an American Society of Lubrication Engineers paper at the ASLE/ASME Lubrication Conference in Kansas City, Missouri, October 3–5, 1977