With the aim of improvement in water quality and the rise in sightseeing value of West Lake, about 11.690 millions yuan RMB was invested in the construction of the water diversion project by national government.After 3~year feasibility studies and over 2~year water diversion experiments, the project from the Qiantang River to West Lake was launched in February, 1985 and completed in September, 1986.The article studied the annual cyclical changes of nitrogen, phosphorus and chlorophylLa concentrations in main parts of West Lake before/after the water diversion.As a result, the seasonal course of total nitrogen concentration which consisted of two peaks occurring in spring and autumn differed from that of pre-diversion project.The concentrations of nitrate in various lake districts all had their peaks in spring (from January to April)t while those of total phosphours appeared in autumn from August to November. An interrelated analysis of samples from various lake districts indicated that the annual cyclical changes of total phosphorus and that of chlorophyll a were closely correlated, but no distinct correlation found between those of dissolved phosphorus and chlorophyll a. As an important but not a fundamental step, the water diversion project did improve the water quality of West Lake in some way, but not so effective as expected.The problem concerning West Lake should be tackled in a comprehensive way, such as pollutants interception, deposit dredgings and pleasure-boat control.