In brown earth, the microaggregates<10 μm and, in particular,<5 μm have a greater reserve of C, N and P, and greater ability for adsorbing N, P and exchangeable cations and more intensive enzyme activity, but a less total content <30%, while the microaggregates >10 μm contain less amount of C, N and P, have a greater ability for desorbing N, P and exchangeable cations, and considerable strong enzyme activity, capecially with a large total content being more than 70%, this fraction of the microaggregates, which in fertile soils is higher than in infertile soils,is one of the important factors influencing the fertility of brown earth. Soil microaggregates can influence the aril physical, chemical and biological properties and may be used as an overall index for evaluating the fertility level of soil. As for soil enzyme activity, it may be used as a supplementary index.