This study was performed with the purpose of determining the efficiency of process of UASB (upflow anaerobic sludge blanket) in treating the wastewater of soft drink industry. The experiment was carried out during 8 months. The cylinder reactor dimensions were 170cm in height with 8 cm internal diameter and total capacity of 8.5 liters. The reactor was inoculated with a mixture of cow wastes, activated sludge of an  industrial wastewater treatment –plant (Zam Zam factory ) and the sludge of an anaerobic digester. The ratio for VSS/ TSS was adjusted at 0.63 , and the loading rate at startup was fixed on 1 kg COD /m³day loading rate .The Reactor was uprated and operated in mesophilic (24.8-38.2 ^ºC) temperature. The experiments then performed in three phases. The first phase of the study was carried out in four steps, the loading rate increased up to 2 kg COD /m³ .day at the constant COD concentration of 2000 mg/l. The efficiency of COD removal was found upto 78% in this phase. In the second phase, and the two steps the loading rate increased up to 2.8 kg COD /m³.day with an increase of COD concentration and 2500 g/m³ .At the end of this phase, the efficiency of COD removal was found to be 78.4 % and equivalent to 0.329 kg COD/kg VSS.day. The detention time was 21.4 hours . At the third phase and in four successive steps , the concentration COD and organic loading rate were increased to 3000 mg/l and 5 kgCOD /m³ .day respectivly .At the end of this phase , the efficiency of COD removal was 78 % (equivalent to 0.389 kgCOD/kg VSS.day). The detention time at this phase was 14.3 hours and the linear velocity of wastewater flow was about 0.12 m/h .Finaly , at the fourth phase of this study the pH of raw wastewater had been increased in few steps up to 10, in order to determine the effect of new conditions on efficiency of the process. The results show that the COD removal efficiency has decreased in the beginning of this phase but again increased to about 78% after adaptation had taken place.The gas generation rate was 0.15 m³/kgCOD removed. Considering the sludge formed in the top of reactor , it could be concluded that the sludge is  flocculant and pellet type.