Abstract: Dynamic changes of biologically active organic fractions of soil such as microbial biomass carbon (SMBC), water soluble organic carbon(WSOC) and dissolved phenols(DP) in a red sand soil as affected by rice straw (RS) and pig manure (MS) were studied. All of these soil organic fractions reached the highest value at seven day after addition of organic manures. Soil microbial biomass and water soluble organic carbon in different treatments decreased gradually with incubation time. However, the contents of dissolved phenols in all treatments nearly keep constant during the whole incubation period. Correlation analysis showed that soil microbial biomass had a significantly positive correlation with both water soluble organic carbon and dissolved phenols,and there was no significant correlation between water soluble organic carbon and dissolved phenols.