The dynamic change of microbial community during sludge acclimation from aerobic to anaerobic in a MBR for coking wastewater treatment was revealed by Illumina Miseq sequencing in this study.The diversity of both Bacteria and Archaea showed an increasedecrease trajectory during acclimation,and exhibited the highest at the domestication interim.Ignavibacteria changed from a tiny minority (less than 1％) to the dominant bacterial group (54.0％) along with acclimation.The relative abundance of Betaproteobacteria kept relatively steady,as in this class some species increased coupled with some other species decreased during acclimation.The dominant Archaea shifted from Halobacteria in initial aerobic sludge to Methanobacteria in the acclimated anaerobic sludge.The dominant bacterial and archaeal groups in different acclimation stages were indigenous microorganisms in the initial sludge,though some of them were very rare.This study supported that the species in rare biosphere might eventually become dominant in response to environmental change.