Organo-mineral fractions of different particle-sizes were isolated from plow layers of 10 paddy soils following removal of light fraction and ultrasonic dispersion in water. As compared with chemical method of dispersion, this method is better in separation of various mechanical separates. Acid hydrolysis of these fractions revealed that, for all the soils ezamined, the proportion of pentoses and neutral sugars increased with the increase of particle size, while the ratio of hezose/pentose and the proportion of NH₄⁺-N in acid hydrolyzates decreased with the increase of particle size. Two trends were observed in the variation of the content of organic C and composition of humus in different fractions of partieal sizes in the same soil. For soils developed on quaternary red clay and weathered product of extrusive rock in the middle subtropical region, organic C in the fraction of particle size<1 p was most abundant, while its molecular weight and aromaticity of humic substances as a whole judged from the humic acid/fulvic acid ratio and the E₄/E₆ of hnmze acid in the fraction were the lowest. For soils developed on the lacustrine deposLts in the northern subtropical regien, the organic matter in the fraction of particle size of 1-2μ was the richest, its molecular weight and aromaticity of humic acid were also the highest.