Objective To know the pollution of microcystin(MC) in Yanghe reservoir during the period of water blooms in the summer of 2007 and to find the variation characteristics about the pollution.Methods The microcystin concentration was determined by ELISA Kit.A 49 consecutive days monitoring was conducted during Jul.-Aug.,2007.Results The concentration of MC in Yanghe reservoir ranged from 0.13 ?g/L to 0.93 ?g/L,and its average level was(0.26 ?0.13)?g/L.As well as,the concentration of MC in Yanghe reservoir descended fluctuant.Conclusion Water in Yanghe reservoir was polluted by MC.All monitoring results of MC concentration during seven weeks(49 days) did not exceed the limit value of microcystin toxin,1 ?g/L for MC-LR,the grade two water ruled in China Environmental Quality Standard for Surface Water.The basic data for changing regularity of MC studied in the present paper will be useful for the warning of alga pollution.