In order to understand the distribution of microorganisms in fluidized-pellet-bed (FPB) bioreactor,bacteria enumeration in different heights (10cm,60cm and 110cm) of FPB was conducted respectively regarding total heterotrophic bacteria,denitrifying bacteria and sulfate-reducing bacteria. Microscopic,scanning electron microscopic (SEM),microbe fixation and embedding techniques were also applied. It was found that aerobic bacteria were dominant in both the bulk solution and pellets along the bed height,and the aerobic bacteria was about 1.6×10⁸ CFU·g^-1 sludge at 10cm height,It can be concluded that aerobic bacteria are playing the leading role of biological degradation in the FPB bioreactor. However,certain amount of anaerobic bacteria were found in the bioreactor and there was a tendency of increase of anaerobic bacteria in the upward direction,It was related to the rapid decreasing of dissolved oxygen (DO) along the bed height. As DO supply was enhanced by increasing the recycling flow ratio,the amount of the aerobic bacteria apparently increased while the amount of the anaerobic bacteria decreased. With single pellet particles,differences were noticed between the outer surface layer and inner cores in number of microbes.