Abstract: The California Bearing Ratio(CBR) denoted the potential strength of subgrade material and was an important index to evaluate its performance in expressway.In order to study the main influence factors of the CBR value of clay,the physical and chemical tests of fifteen groups of subgrade materials from the Qingdao-Yinchuan Expressway in Hebei Province were done.Based on the result of tests,that there was obvious difference between CBR values of clays with the same plastic index,the influence factors of CBR of expressway subgrade materials were studied,such as distribution of particle sizes of soils,mineral component and clay mineral component,and useful conclusions were gained.The main influence factors of CBR of expressway subgrade materials were the type and content of minerals component and the shape of distribution curve of particle sizes of soils.The CBR value decreased with the increase of clay-silt ratio.