Abstract: Phosphorus loadings in sediments play an important role in lake eutrophication and the progress of its recovery. The Phosphorus release is controlled by physical ,chemical and biological mechanisms. Alkaline phosphatase catalyzes remineralization of organic phosphorus and then it may be an important factor accelerating Phosphorus cycling in sediments. In this paper ,we discussed distributions and roles in the phosphorus release of alkaline phosphatase in sediments of some lakes in the middle and lower reaches of the Yangtze River. Alkaline phosphatase activity (APA) in surface sediments in the dredged and undredged zones of the Lake Wulihu and the different areas of the Lake Taihu ,Chaohu ,Longganhu ,Donghu ,Yuehu ,Longyanghu and the Lianhuahu are quite different . The spatial heterogeneity is related to the degree of eutrophication. Moreover ,APA decreased with the sediment depth and may peak in the intermediate and deeper layers showing a seasonal variation. The enzyme differently responded to its inhibitors. All the facts implied that the enzyme occurred in various forms (isoenzymes) . L-Phenylalanine (Phe) enhanced APA in the sediments of the Lake Wulihu and the Lake Yuehu. Reacted with Phe and incubated for one day ,the sediments released the bioavailable phosphate (SRP) with significantly higher concentration ,while it inhibited APA ,in the sediments of the Lake Yuehu. Reacted with Phe and incubated for one day ,the sediments released significantly larger amounts of dissolved organic phosphorus ,without increasing the SRP concentration. Hence ,the SRP released resulted ,at least partially ,from the hydrolysis of some liable organic phosphorus mediated by APA. Alkaline phosphatase in lake sediments plays an important role in the release of internal P loadings and eutrophication.