The purpose of this study was to determine the efficiency of the USBFprocess for biological nitrogen removal and achieving the optimal hydraulic retention time according to the maximum nitrogen removal in the system. The synthetic wastewater which was used in this research, was a compound of glucose and urea as a carbon and nitrogen source with the COD/N ratio remaining around 10, while the N concentration based on the sum of nitrite and nitrate ions was approximately 50 mg/L. The investigation was carried out over two phases of start up with inoculation, and the operation at differing hydraulic retention times with flow rates of 0.8, 1, 1.2, 1.4, 1.6, 1.8 and 2 lit per hour. The experimental results showed the increasing efficiency of the system in nitrogen removal with increased flow rates, such that 89.4, 92 and 95.1 percent of the initial phosphorus concentrate in the wastewater was removed at flow rates of 0.8, 0.1 and 0.12, respectively; however, at flow rates of 1.4, 1.6, 1.8 and 2 lit per hour, the mean nitrogen removal efficiency was reduced to 93, 88, 83 and 80 percent, respectively. At the end, the hydraulic retention time for the biological nitrogen removal in this process, was determined at 40 hours in the whole system which is equivalent to 5 hours hydraulic retention time in the clarifier. USBF, as an advanced biological process, had an appropriate removal efficiency for the biological treatment of nitrogen.