The relation between the fractional composition of phosphorus forms in sediment and phosphorus release was investigated experimentally using incubation bottles. The validity of the sediment phsophorus fractionation method developed in the field of soil science was examined and it proved to be successful for fractionation of pure phosphate compounds which were added to sediments. The iron and aluminum phosphate and adsorbed phosphorus were recovered in NaOH (lN) extractable ortho-phosphate (NaOH-o-P) fraction, calcium phosphate in HC1 (lN) extractable ortho-phosphate (HCl-o-P) fraction, and organic phosphorus in MaOH extractable total-phosphorus except for ortho-phosphate (NaOH-[T-o]-P) fraction and residual (Res.-P) fraction. This method was used to identify the form of phosphorus in sediments collected from a lake and a canal. NaOH-o-P fraction was the main phosphorus constituent in both sediments. The change in MaOH-o-P fraction accounted for most of the change in total sediment phosphorus. NaOH-o-P fraction in the sediment decreased during the period of incubation as the concentrations in the water increased but other fractions remained constant. The addition of iron phosphate and adsorbed phosphorus to the sediments promoted phosphorus release. Therefore, the changes in the fractional composition of the phosphorus forms indicated that NaOH-o-P fraction is the form of phosphorus which is most easily released under anaerobic condition. The amount of this fraction is the index of the phosphorus release potential of sediments.