Seasonal variation in phosphorus release potential from bottom sediment and environmental factors affecting phosphorus mobilization in shallow eutrophic lakes, Lakes Nishiura and Kitaura in Japan, were investigated. At uplake sites, the fluxes were low and showed no seasonal variation. The Eh in the sediments at these sites was positive and in an oxidizing state, whereas sediments at other sites were in a reducing state. The oxidation/reduction states were not affected by the presence of DO in the boundary layer water. DO in the boundary layer only oxidized the surface of sediment layer and affected DIP release from the sediment. The oxidation of sediment was due to nitrate input from inflowing rivers. At mid-lake sites, where the DIP release flux was high, the Eh in sediments was increased by nitrate input. Nitrate, which is an electron acceptor in the boundary layer, diffused into the sediment and oxidized the sediment layer in contrast to the definitive oxidation attributed to DO. The DIP release flux decreased with increasing nitrate supply rates. The sediment record shows that the oxidation state was maintained by nitrate supply, which is affected by the decomposition rate of nitrate and strongly affects DIP release from lake sediments under anaerobic conditions.