Abstract: There are differences between the methods of Biscaye (1965) and Cook (1975) et al used for calculating relative clay mineral contents.The former is usually used in the analyses of oriented slides, while the latter is suitable to deal with the random powder X-ray diffraction mounts.In this study,main types of clay minerals (i.e.smectite,illite,kaolinite and chlorite) in <2 μm fraction sediments of the 7 surface samples from the Yangtze Estuary and 258 surface sediment samples from the continental shelf of the East China Sea were analyzed.The <2 μm fraction samples were extracted based on the Stokes' law and made up the oriented slides.Of 7 sediments from the Yangtze Estuary,each <2 μm fraction was prepared for both oriented slide and powder samples.All of the <2 μm samples were measured with X-ray diffraction and the software of TOPAS 2.0.Then the contents of main types of clay minerals were determined by both Biscaye's and Cook's methods.The analysis results show that the Cook's method can be used in dealing with oriented slides diffraction mounts,because the results of slides and powder samples have no significant discrimination with using Cook's calculation method.Statistical analysis shows some relationship between two group contents which are calculated with Biscaye's or Cook's method respectively,such as the relative contents of smectite and illite.According to the linear relationship coefficients between the two group contents,we build the conversion formulae for them,which can be used in the study area of the East China Sea shelf.