Abstract: According to the data on water quality monitoring of seawater in Xiamen Bay in August 2010,this paper illustrated the variation trend of nitrogen and phosphorus nutrients and their mutual relations,and discussed the eutrophication degree of the sea in Xiamen Bay.The results show that the distribution trend of surface COD,DIN and PO4-P is roughly progressively decreasing from western sea of Xiamen,Maluan Bay and Jiulong River Estuary to outside the bay;Correlation analysis showed that the main source of COD,DIN and PO4-P in the investigated sea was the input of nutrients from land-based emissions along the coast of Xiamen Bay and Jiulong River;The sources of COD,PO4-P and NH4-N might be the same,which had similar biogeochemical process.While the main source of NO3-N might be Jiulong River.The variation range of N/P value of Xiamen Bay was 2.26~117.30,and the values of N/P in most areas of Jiulong River Estuary,southeastern sea and Dadeng sea area were greater than 16. According to the N/P value,the structure of nutrients was phosphorus-restricted,while the north of western sea area,Tongan Bay and Maluan Bay belonged to nitrogen-restricted sea areas.The range of the eutrophication index of the sea area in Xiamen Bay(E)was 0.013~118.168,while the eutrophication indexs of Maluan Bay,the north of western sea and the sea near Gaoji Seawall were higher.