Abstract: In this paper, humic acids from cumulic cinnamon soil w ith different fertilization treatment ( no fertilizer, chemical fertilizer, co rn straw and manure) for 17 years were used fo r studying the complexation characteristic w ith iron on the basis of studying the properties of the humic acids.The main results show ed that complex ability of humic acid from organic material treatments ( corn straw , manure ) decreased comparing w ith that of chemical fertilizer treat- ment and no fertilizer treatment, w hich w as caused by the low content of total acidity and car- box yl.Complex ability of humic acid may be affected by the factor of pH, temperature and ion strength.Complex stability constant ( logk) increased as the pH rose, w hile decreased w ith the increase of ion strength and temperature.The complex reaction of humic acid w ith iron w as an exothermic reaction