UASB reactor was operated for treating wastewater containing high sulfate to assess their performance, competition between sulfate-reducing bacteria(SRB) and methane-producing bacteria(MPB), and the change in the characteristics of microbial granules according to change of hydraulic retention time(HRT) in the reactor. The reactor was fed with a synthetic moderate strength wastes(glucose, 2000 ㎎COD/l) containing high sulfate(2400 ㎎SO_4^2-/l). The organic loading rate(OLR) ranged from 1.5 to 3.0 gCOD/l.d as HRT maintained 15 to 30 hrs in the stage Ⅰ. The COD removal efficiency was between 80 to 92%. During this period, methane yield rapidly decreased from 0.3 to 0.1 1CH_4/gCODremoved. While sulfide concentration in the effluent increased from 80 to 200 ㎎S/l. This indicates that SRB becomes dominant over MPB at a relatively long HRT in the excess sulfate. When OLR of reactor maintained from 5 to 8 gCOD/l.d in the stage Ⅱ, methane yield increased from 0.1 to 0.17 l CH_4/gCODremoved regardless of decrease of COD removal efficiency. This indicates that SRB is more sensitive to the change of a short HRT than MPB. In the competition between SRB and MPB, about 30% of the removed COD was utilized by SRB at HRT of 30 hrs during the start-up period, while about 73% was used by SRB at HRT of 15 hrs at the final step of second experimental stage. Whereas after shock exposure of OLR about 62% was utilized by SRB at HRT of 5 hrs. It indicates that SRB is strongly suppressed by the wash-out of significant dispersed SRB since a large electron flow is distributed to the MPB. In addition, the granulation in the presence of high sulfate is unfavoured at a long HRT because of substrate transport limitations into MPB like Methanothrix spp. which is an important factor in the composition of the granules. Accordingly, granule sizes in the UASB reactor decreased with time due to weak network frame of granules by the decreased activity of MPB.