Abstract: With rapidly industrial and agricultural development, more and more fertilizers, chemicals and heavy ions etc. flow into lakes and rivers without cleaning, which causes lake eutrophication, decreasing quality in drinking water sources. Therefore, denitrification is essential for controlling the amounts of nitrogen. During the process from nitrate to the end productsnitrogen, several intermediates, e.g. nitrite (NO2-), nitrous oxid(N2O) and nitric oxid(NO) may be accumulated which have more toxic influences on environment. In this study, denitrification effect of microorganisms was examined on the changes between oxic and anoxic conditions under varying pH. At pH 7.5, denitrification proceeded well after 3 switches from oxic to anoxic condition and vice versa. Production of N2 was constant and amounts of NO2-, N2O and NO were little. However, at pH 6.8, the denitrification activity was inhibited and there were a large amounts of intermediates. Dry weight of denitrifying bacteria decreased violently and washed out.