Abstract: Microstructures of flaws of ball bearings are investigated by using field emission scanning electron microscope and it is found that both crack initiation and crack propagation are always along a certain curved surface. Propagating to a certain degree, the crack will fracture and form an elliptic flaw. Along with the ball bearing operating, not only the flaw continues to flake away, but also some new cracks initiate and propagate along new curved surfaces and fracture ultimately. There are many cross sections in the flaw. Based on this phenomenon, the critical curved surface of ball bearing is proposed and crack initiation and propagation are assumed to be on this critical curved surface. In virtue of this method, three dimension problems are converted to two dimension ones. Based on this, a modified Paris fatigue life prediction model is proposed. The fatigue life is divided into two sections：crack propagation and flaw size gain, and fatigue life are predicted by sectional Paris law. The model is validated by fatigue test life data of SKF 6205 deep groove ball bearings. In comparison with the prediction of traditional SKF L-P formulary, the proposed model produces more accurate results.

Key words: Ball bearing, Critical curved surface, Fatigue life prediction, Paris law, Prediction model