Abstract: Geometry number was introduced in this paper.According to the geometry relationship of rolling element bearing and the statistics of 1274 bearing models,it was found that bearing geometry number of bearings of the same series of is relatively constant,but it is quite different for different series of bearing.Geometry numbers of 45 series can be classified to 7 basic types.A method to estimate characteristic octave frequencies of defect in inner race and outer race was introduced.The method is based on number of balls,the geometry number and series of bearing.Since geometry numbers are provided and the number of balls and the series of bearing can easily be obtained in practice,the method is helpful for detecting defects in bearings.