In order to determine the effects of bedrock, organic matter, calcium and iron oxide on the soil aggregation, th:.s research has performed with soils from bedrock regions of Limestone, Granite and Granite gneiss. This research was also to estimate how organic matter, calcium and iron oxide influence on soil aggregation under different forest conditions in various bedrock regions. And it also had a purpose to rate physical factors relevant to soil aggregation, their characteristics and aggregate diameter which closely relates to staibilities in the process of soil erosion. The following conclusions have been drawn in response to the overall research objectives. The rates of the soil aggregation on different bedrock regions were 21% in Limestone bedrock, 19.6% in Granite bedrock and 9.9% in Granite gneiss bedrock. A main factor in soil aggregation was the orgainc matter content in soils and the rate of soil aggregation increased in the constant proportion with the organic matter content. The relation could be formulated into Y=4.31-4.37(Y) : aggregation ratio X : organic matter content. The soil aggregation ratio under the deciduous forests eras higher than that under the coniferous forests. It was considered that this resulted from differences in organic matter content. Soil aggregates with larger diameter than 0.5㎜ were found more in Limestone bedrock than other smaller size soil aggregates of 0.25㎜ diameter were more distributed in Granite gneiss bedrock. Granite bedrock region had normal distribution in soil aggregate sizes with the highest frequency of 0.5㎜ diameter. Calcium and iron oxides had only partial influences on the soil aggregation in some specific conditions. But in Limestone bedrock region calcium influenced on the soil aggregation with the organic matter content,