This study focused on the change of bacterial communities from a new denitrificatin technology where corncob was used as carbon source and the only physical support for microorganisms, under different dissolved oxygen (DO) conditions. The polymerase chain reaction (PCR) and denaturting gradient gel electrophoresis (DGGE) technique was used for extracting DNA from biofilm samples which collected from corncobs. The sequences of several 16S rDNA DGGE fragments were determined, and the dominant bacterial species that were present in each sample were proposed after comparing the results with data of the NCBI Gene Bank. The results indicated that PCR-DGGE technique was successfully used in this study. Clear relationship was found between DO condition and dominant bacterial communities during the experiment. Facultative anaerobic microorganism such as Bacteroidetes and Clostridium were found as dominant bacteria when DO was lower than 1mg/L. And aerobic microorganism such as Bacillus became the dominant bacteria when DO in the water was high than 2mg/L. These bacterial species mentioned above are all main types of denitrifying bacteria reported in other researches. And the results are helpful to explain the phenomenon why high nitrate removal efficiency was always found when DO condition changed. In addition, the results provided some valuable references for studing bacterial communities under different DO conditions.