In this study a 3.3 m pilot-scale UASB reactor was operated to investigate the performance of an inner tube (IT)-type gas-solid separator (GSS) device, as well as the effects of the superficial gas velocity on the hydrodynamic characteristics of an upflow bioreactor The sludge hold-up efficiency of the IT-type GSS device increased gradually as the superficial gas production rate increased. When the superficial gas velocity was 0.26 m/h, the UASB reactor excluding the settling compartment was described as a non-ideal plug flow with a dead space of about 20%. However; when the superficial gas velocity increased to 0.67 m/h, the overall flow pattern was close to a completely stirred tank reactor (CSTR), without significant difference inflow pattern in the settler.