In order to determine the influence of the molecular weight distribution of lubricating oils on the fatigue life of ball-bearings, four types of paraffinic mineral base oils and four types of blended oils prepared by mixing two base oils were used to evaluate the life of 187 radial ball-bearings (reference number 6206) at temperatures of 90–121°C. The data showed that the blended oils with a wide range of molecular weights allowed a fatigue life up to about three times longer than the base oils with a narrow range of molecular weights. From the fatigue life analyses, colour variation of the bearings, measurements of electrical resistance of the oil films etc., the present authors expect that this difference in the fatigue lives may be mainly due to the production of a surface-protecting film when the elasto-hydrodynamic lubrication oil film was partially broken.