【Objective】Iron oxides are important cementitious substances in soil aggregates, which mainly exist in soil clay particles. This study aimed to reveal the transformation of iron oxides in soil aggregates and their enrichment characteristics in clay fractions of different particle sizes under the conditions of returning Chinese milk vetch to the field and applying chemical fertilizers.【Method】Four treatments including a single application of Chinese milk vetch (MV), a single application of chemical fertilizer (F100), Chinese milk vetch applied with chemical fertilizer (MV+F100) and Chinese milk vetch combined with 80% of chemical fertilizer (MV+F80) were selected. Soil aggregates (>250, 53~250 and 2~53 μm) and clay particles (<2 μm) were separated by wet sieving and centrifugation, and the content of iron oxides in soil aggregates and their clay particles with different treatments was determined. Also, the relationship between iron oxides and the stability of aggregates was analyzed.【Result】(1) Compared with MV and F100, the content of >250 μm aggregate was increased and the content of 53-250 μm aggregate was decreased, and the mean weight diameter (MWD) and geometric mean diameter (GMD) were increased by 8.2% to 20.5% and 4.1% to 8.5%, respectively. (2) Compared with MV, the combined application of Chinese milk vetch and chemical fertilizer significantly increased the contents of amorphous iron (12.7%-55.6%, 52.4%-54.9% and 45.9%-48.6%) in aggregates of various sizes, clay particles of 2~53 μm and free clay particles. The enrichment rates of amorphous iron in >250 and 53~250 μm aggregates were decreased (32.8%~36.8% and 17.2%~28.4%). Compared with F100, the content and enrichment rate of amorphous iron in 2~53 μm aggregates and clay of aggregates were significantly reduced (3.5%~21.3%, 29.2%~30.4% and 10.9%~26.9%). (3) The content of >250 μm aggregates was positively correlated with the content of non-crystalline iron and activation degree of iron in the clay of aggregates, and positively correlated with the enrichment rate of amorphous iron and the content of free iron in aggregates. The MWD and GMD were significantly positively correlated with the content of >250μm aggregates, and significantly negatively correlated with the content of <2μm aggregate. The free iron in the clay of >250 and 2~53μm aggregates and free clays was the most important factor influencing the MWD and GMD.【Conclusion】Combined application of Chinese milk vetch with chemical fertilizer could increase the content of iron oxide in the clay of different aggregates and promote the formation of >250μm aggregates, thus, enhancing the stability of aggregates.