The occurrence of harmful algal bloom in water source poses a serious water safety problem to local water supply systems. In order to ensure the raw water quality, the feasibility of an in situ light-shading measure was investigated through enclosure experiment and pilot-scale experiment. The results showed that harmful algal bloom could be controlled by light-shading lasting for 6-9 days, with water quality being partially improved. When aeration was added, the reduction of algal biomass could be enhanced, and water quality was further improved compared to that without aeration. These experimental results offered an attractive in situ algal control measure for lakes or reservoirs suffered from harmful algal bloom. (C) 2008 Elsevier Ltd. All rights reserved.