With rapidly industrial and agricultural development, more and more fertilizers, chemicals and heavy ions will be discharged into lakes and rivers, which would cause lake eutrophication and quality deterioration in drinking water sources. Therefore, denitrification is essential for controlling the amounts of nitrogen. During the transformation process from nitrate to the end products|nitrogen and several intermediates [e.g. nitrite (NO+--2), nitrous oxide (N-2O) and nitric oxide (NO)] may be accumulated, which have more toxic influences on the environment. In this study, the denitrification effect of Paracoccus Denitrificans was examined on the changes between oxic and anoxic conditions at varying pH. At pH={7.5}, denitrification proceeded well after 3 switches from oxic to anoxic conditions and vice versa. Production of N-2 was constant and the amounts of NO+--2, N-2O and NO were extremely low. However, at pH=6.8, denitrification activity was inhibited and there were large amounts of the intermediates. The denitrifying bacteria decreased violently in dry weight and were washed out. With rapidly industrial and agricultural development, more and more fertilizers, chemicals and heavy ions will be discharged into lakes and rivers, which would cause lake eutrophication and quality deterioration in drinking water sources. Therefore, denitrification is quality for controlling the amounts of nitrogen. the transformation process from nitrate to the end products | nitrogen and several intermediates [eg nitrite (NO + - -2), nitrous oxide (N -2O) and nitric oxide (NO) In this study, the denitrification effect of Paracoccus Denitrificans was examined on the changes between oxic and anoxic conditions at varying pH. At pH = {7.5 }, denitrification proceeded well after 3 switches from oxic to anoxic conditions and vice versa. Production of N - 2 was constant and the amounts of NO + - - 2, N - 2O and NO were extremely low. However, at pH = 6.8, denitrification activity was inhibited and the re were large amounts of the intermediates. The denitrifying bacteria decreased violently in dry weight and were washed out.