Abstract: In order to reveal the succession of microbial community diversity in a hybrid membrane bioreactor (MBR) and provide microbial evidence for technique improvements,total microbial DNA is extracted from six samples which are collected three times respectively from mixed liquor and biofilm of a MBR,16SrDNA fragments were amplified from the total DNA successfully using a pair of universal bacterial 6SrDNA primer,GC968F and 1492R,and then the amplified PCR are used for denaturing gradient gel electrophoresis (DGGE) analysis.DGGE analysis indicated that a noticeable change takes place in microbial structure at initial stages of MBR operation．In the whole operation,bacterial community structures are higher.In this bioreactor,because of the existence of filling，there might be two different modes of enitrification in the reactor because there might be anaerobic environment in it.Two months later,some increasing bacteria has greater impact on the production and accumulation of membrane pollutants.