Abstract: The differences of P adsorption-desorption characteristics of amorphous and crystalline iron oxides and their(relation) to P adsorption saturation were studied by using three synthetic iron oxides.The results showed that P adsorption properties of these three synthetic iron oxides could be described by Langumir equation with a correlation coefficient larger than 0.9 being statistical significant at 1% level.It was found by comprehensively taking Qm(maximum quantity of adsorption),K(adsorption constant) and MBC(maximum buffing capacity) into account,ferrihydrate(amorphous) was much larger than crystalline iron oxides(goethite and hematite) in both intensity and capacity of P adsorption.P adsorbed by ferrihydrate was much more difficult to be desorbed than those by goethite and hematite.The large active surfaces of ferrihydrate contribute little on P desorption.It was suggested by our results that P adsorption saturation might be a promising integrated index for estimating the intensity and capacity of P adsorption-desorption in soils or iron oxides.