To improve the efficiency of petrochemical wastewater purification, the relationship between bacterial community structure and pollutants loading/degrading rates in A/O process for petrochemical wastewater treat-ment was investigated by denaturing gradient gel electrophoresis(DGGE)of the 16S rRNA gene fragments am-plified by polymerase chain reaction(PCR). Results show that while the influent COD and NH4~(+)-N concentra-tions are 425.92-560 mg/L and 64-100 mg/L respectively, the corresponding average concentrations of the effluent are 160 mg/L and 55 mg/L, which are 1.6 and 3.6 times more than the national standards respective-ly. It demonstrates that the performance of pollutants removal process is inefficient. The analysis of PCR-DGGE profile indicates that the bacterial community structure of the activated sludge in A/O system is species-rich but unstable, and the highest and the lowest similarity coefficients are 36% and 6. 25% respectively, which shows that remarkable community structure evolution exists in the system. The variation of bacterial community struc-ture and pollutants loading influences the removal efficiency of pollutants obviously, and relatively stable com-munity structure leads to the stable operational performance of biological wastewater treatment system.