Grain-size distributions of ancient paddy soil (n = 100) and Xiashu loess (n = 105) were measured by laser diffraction and sedimentation techniques, the measured results of the two methods were not comparable for soil samples from the vertical-profile series, but comparable for topsoil samples with different textures. Significant correlations between clay fractions and sand fractions existed in the two methods (r = 0.98 and 0.93, respectively). Both clay and sand fractions obtained by laser diffraction were lower than those by sedimentation techniques, while silt fractions obtained by laser diffraction were clearly higher than those by sedimentation techniques and no significant correlation between the two methods was found. Neither of the two methods can get the “real” grain-size distribution, the results represent different grain-size characteristics of the same sample.