Structure of a production water microbial community in a Daqing petroleum reservoir was investigated by T-RFLP (Terminal Restriction Fragment Length Polymorphism) and clone libraries approaches. Small subunit rRNA gene (16S rDNA) was retrieved from the samples sampled in July and October 2005. T-RFLP analyses were conducted with the results that the archaeal community was less diverse than the bacterial community. Time change resulted in the shift of microbial community structure; however, the structure of the archaeal community was not remarkably different between July and October 2005. In contrast, the patterns of bacterial community showed significant difference in these two months The archaeal 16S rDNA partial sequences were mainly affiliated with Methanosarcinales and Methanomicrobiales. The bacterial phylotypes were very closely related to the “Betaproteobacteria” “Gammaproteobacteria” “Deltaproteobacteria” “Epsilonproteobacteria” , Bacteroidetes, and Deferribacteres as well as a deepbranching microbial cluster which was predominant microorganisms in the production water from October 2005.