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The microbial communities in 6 granular sludge samples taken from a lab-scale EGSB reactor operated under different organic loading rates (OLR) at 20 degrees C were studied using PCR-DGGE and RTQ-PCR technique. The results indicate that as the OLR increased from 1.5 kg/(m3 x d) to 10.0 kg/(m3 x d), the microbial communities of archaea and eubacteria in the 6 sludge samples are not changed greatly. The archaea population is highly similar among the 6 samples. The Shannon indexes of archaea population are 5.51, 5.88, 5.47, 5.25, 5.32 and 5.11, respectively. Except the seed sludge, the eubacteria population is similar among the other 5 samples. The Shannon indexes of eubacteria population are 2.97, 5.07, 5.44, 6.38, 6.66 and 5.21, respectively. The dominant archaea in the 6 granular samples are Methanobacterium, Methanocorpusculum, Methanosaeta and Methanospirillum. As the OLR of the reactor increased and the operation time elapsed, Methanocorpusculum parvum and Methanospirillum hungatei, both are hydrotrophic methanogens, become the dominant archaea. The archaea content in the samples decreases slightly at the beginning, but increases later and finally reaches to a level obviously higher than that in the seed sludge.