This study aims to investigate, the effect of organic loading rates (OLRs), nutrient ratio addition, and sludge retention time (SRT) on treating dairy wastewater in a sequencing batch reactor (SBR) system. This investigation is verified by experiments conducted in 3 phases at 3 different OLRs (1.8, 1.2, and 0.9 kg/m3d, respectively).  Urea ((NH2)2CO) is added to make a suitable (COD:N:P) ratio of (100:5:1) in dairy wastewater. The SRT is adjusted from 50 days to an appropriate value of 18 days. The obtained results show that the COD, TN, and TP removal efficiencies are increased with decreasing OLRs. Sludge concentration in the SBR tank is stable at 1100 mg/L after adding (NH2)2CO. In addition, the SBR operated at a suitable SRT (i.e. 18 days) helps the biomass stably, resulting in enhancement of COD, TN, and TP removal. The results are helpful to the design of SBR for treating dairy wastewater.