Abstract: The simultaneous aerobic nitrifying-denitrifying process was investigated using the mixed culture method.The pH value of 7～10 was the favorable condition for the growth of mixed denitrifying bacteria.To obtain high cell concentration during the cultivation using 5 L tank,pH was controlled below 8 at the initial fermentation stage,and uncontrolled afterwards.The dry cell weight reached 3 9 g/L,increased by 62 5% compared to the value in the natural pH culture conditions.Further scale-up culture was investigated in 10 L fermentation tank.The biomass was further increased by 15 4% by the feeding of ammonium sulfate,and the maximum biomass was 4 5 g/L.