The present paper deals with the change in organic matter and its role in aggregarion in uhe semiarid ridge culture and rotation of paddy rice and upland crops under different fertilization conditions. The results indicated that the contents of organic carbon in soil, carbon in heavy fraction, loosely combined humus and stably combined humus were obviously increased，when the pig manure was applied. The amounts of organic carbon, loosely combined humus and stably combined humus in soil were decreased gradually as time went on. On the contrary, the carbon in heavy fraction, tike quantity of complexation and the degree of complexation were raised. The semiarid ridge culture was beneficial to the accumulation of organic carbun, carbon in heavy fraction and loosely combined humus. The loosely combined humus and stably comtined humus were beneficial to the formation and stabilization of > 0.25mm aggrebates. The tightly combined humus was iimpottant for the stabilization of microaggregation.