The annual fluctuation of chlo rophyll-a concent rations and some chemical indicators were measured in the main areas of West Lake, Hangzhou during January-December 1999.The algal grow th potential test in water from West Lake were made.The relations between the chlo rophylla concent rations and various ecolog ical factors were discussed.The experimental results are as follows.The concent rations of chlorophyll-a in main Lake-areas appear in the same level.The range was 41.16-191.26mg/m³ and the annual average concent ration was 99.98mg/m³.The concentrations of chlo rophyll-a showed marked seasonal variation the highest in summer and early autumn and the low est in winter.The concent ration of chlorophyll-a has close relation with water temperature and TP.The West Lake is a typical Cyanophyta-pat tern lake in which the annual average concent rations of TN and TP were 2.08mg/L and 0.121mg/L respect ively.The ratio of TN to TP is more than 17 in West Lake, indicating TP's domination in the stimulation of algal growth.