<p>Bacillus species is an important microbial species in the activated sludge process. Some researchers reported that the predominance of Bacillus spp. improves treatment performance and good sludge settleability. The viable count of Bacillus spp. is usually measured by the plate culture method. Recently developed massive sequence technology has been applied to activated sludge samples; this technique provides more detailed information on its microbial community. However, the relationship between the number of Bacillus spp. and microbial compositions is not yet well understood. In order to elucidate this relationship, microbial community analyses based on 16S rRNA gene sequence and cell count of Bacillus spp. were conducted. Activated sludge samples, including those from conventional activated sludge, sequencing batch, and oxidation ditch processes, were collected and subjected to analysis. The results of microbial community analysis revealed that Proteobacteria and Bacteroides were the predominant bacterial phyla and overall community compositions resembled each other at the phylum level. The detection ratio for Bacillus spp. was 0–0.33%, and the number of Bacillus spp. ranged from 10⁵ to more than 10⁸ colonies/g-MLSS. The results showed that the number of Bacillus spp. and detection ratio showed a similar trend, and thus, these analyses could be complementary to each other.</p>