It is well known that the organic matter content in the soil has an influence on the formation and stabilization of soil structure. In this paper, taking over past researches, the author carried out the study as follows : In the preliminary investigation, organic matter in the soil was classified according to the degree to which organic matter decompose. Then as a quantitative assay for readily decomposable organic matter, 150℃～200℃ heat treatment and 6～30% H_2O_2 oxidation treatment were proposed. Next, as for the method to express soil structure, though at present, soil structure is generally estimated quantitatively by three phases of soil, the fourth phase expressed organic matter content may proved to be sometimes accepted as effective relating it to physical properties of the soil. In the further experiment, contact angle between water and soil was measured by geometric method and it is proved that the contact angle tend to increase with organic matter content in the soil.