Soil organic matter is the basis of soil fertility, and also controls the balance of CO₂ in the atmosphere and the environmental behavior and biological effectiveness of nutrients, heavy metals and persistent organic pollutants (POPs) in Earth's surface environment. Therefore, it is necessary to pay attention to the process of stability of organic matter. This paper dealt with the mineralogical mechanism of soil organic matter stability by studying the crimson soil profile in Conghua, Guangzhou City. The following processes were carried out: determination of the basic properties of the soil, including pH and mechanical components; determination of total organic matter content and stable organic matter content by potassium dichromate oxidation; determination of the mineral stable organic matter content by TOC analyzer; extraction of free iron oxide with dithionite-sodium citrate; extraction of amorphous iron oxide with ammonium oxalate buffer; determination of iron content by atomic absorption spectrophotometry; XRD semi-quantitative determination of the composition and content of clay minerals; and the correlation analysis between organic matter and mineral. The results indicate that the content of total organic matter, stable organic matter, mineral stable organic matter and recalcitrant organic matter is reduced with the increase of depth. Mineral stable organic matter is mainly existent in the middle layer of soil, while iron oxide minerals mostly occur in the upper soil. The content of kaolinite is dominant (58.7%～84%), the content of smectite is relatively high (8.5%～16.8%); besides, there exists much gibbsite. The content of smectite decreases with the increasing depth, whereas the content of kaolinite and gibbsite increases with the decreasing depth. The correlation between organic matter content and iron oxide mineral is much higher than that between clay mineral and gibbsite. Soil organic matter content and amorphous iron content are much better than the free iron content in the correlation. For instance, there is positive correlation between organic matter and amorphous iron (R²>0.9), there is positive correlation between organic matter and smectite (R²=0.6971),and there is negative correlation between organic matter and kaolinite, gibbsite . Obviously, the amorphous iron plays a vital role in the stabilization of soil organic matter; nevertheless, the specific surface area of amorphous iron will limit the capacity of the soil C sequestration, and the adsorption capacity will gradually become smaller until saturation. So we should pay more attention to the mineralogical mechanism of soil organic matter stability and influencing factors.