Three representative types of paddy soil (well permeable, side bleached and gleyed) and three kinds of plant material(azolla, rice straw and milk vetch)were used in the experiment. The results obtained are summarized as follows: The content of 1.0-0.25 mm water-stable aggregates in the paddy soil of high-fertility ranged from 2-14% greater than that in the paddy soil of low-fertility. The destruction perecentage of structure and the degree of dispersion were higher for the infertile paddy soils. Soil organo-mineral complex is an important factor in determining soil fertility and it is closely related to the structure of paddy soils. It was found that about 80-90% of organic matter in paddy soils were associated with clay fraction, forming organo-mineral tomplexes. The quantity of comple."ted organic carbon was significantly correlated with the total content of organic carbon of the soil and aggregates (Fig. 3).the quantity and degree of additional complexing of organic matter (QAC and DAC) increased with the raising of fertility of the soil (Table 6).If the amount of organic material added to the soil less than 1% of the dry soil weight, the largest coefficient of complexing (CC) might be obtained(Fig. 7). According to the preliminary investigation of ‘QAC', ‘DAC' and ‘CC', the effect of the organic materials on the soil structure improvement is in the following order: Azolla>Rice straw>Milk vetch (Fig.4-6).