Abstract: The microbial community structure and functionally distinct groups in three kinds of produced water samples from the shallow, mesothermic and low-salinity Daqing oil reservoir were systematically evaluated using both culture-dependent and culture-independent methods. Sequence analysis of the 16S rRNA genes indicated that the bacterial library was dominated by Acinetobacter and Arcobacter and the archaeal community was dominated by Methanosaeta and Methanolinea. Two isolated methanogens were closely related with Methanothermobacter thermautotrophicus and Methanoculleus receptaculi. The fermentative bacteria were identified as Pseudomonas, Haloanaerobium, Alcalibacter, Arcobacter, and Pannonibacter. The predominant nitrate-reducing bacteria fell within the genus Pseudomonas. The dominant members of the cultured hydrocarbon-oxidizing bacteria were phylogenetically associated with Micrococcus, Pseudomonas, and Bacillus. Enrichments of biosurfactants and biopolymer producing groups mainly yielded Pseudomonas, Bacillus, and Acenitobacter-related members. The functional groups related to polymer degradation were also affiliated with Pseudomonas and Bacillus. Results from this study provide the fresh insight into the diversity of microbial communities in Daqing petroleum reservoirs. The vast pool of functional strains retrieved in this study was presumed to include the promising strains that could be applied in microbial-enhanced oil recovery in future.