The purpose of this research was to study the effect of temperature at 5, 15, 25, 35 and 40 ํC on the efficiency of biological phosphorus removal (BPR) by an Anaerobic/Aerobic SBR process. The synthetic wastewater with COD, TKN and P of 300, 15 and 15 mg/l (100:5:5), respectively, was used to promote the proliferation of Phosphorus Accumulating Organisms (PAOs) but not nitrifiers. The experimental setup included two 16.8 l automatic temperature-controlled SBR reactors. The sludge age was controlled at 12 days and the cycle time was 12 hrs. (4:50 hrs. anaerobic, 6 hrs. aerobic, 1 hr settling and 10 minutes influent feed and effluent withdrawal). The activated sludge from the Si Phraya sewage treatment plant and the pure culture of Pseudomonas fluorescens and Acinetobacter calcoaceticus were cultivated for approximately 7 months after which PAOs were in abundance. The said mixed culture was then acclimatized for 1-1.5 months with the synthetic wastewater and operated at the designated temperatures before the process performance was investigated. Apparently, the temperature directly affected the phosphorus removal efficiency, i.e., at 5, 15, 25, 35 and 40 ํC, the P removal efficiencies dropped from 100, 100, 100 and 72 to 61 percent, respectively. The P content in mixed liquor volatile suspended solids (MLVSS) was 10.8, 10.4, 5.5, 3.1 and 3.5 percent for the same cases, respectively. This clearly showed that the PAOs proliferated better at lower temperatures. The temperature had, however, less impact on the filtered COD removal efficiency; it was apparently 99, 99, 97, 99 and 93 percent, respectively. This is due to the sufficiently long HRT and SRT (12 days overall and 5.5, 6.5 days for the anaerobic and aerobic stage, respectively) of the systems. The excessive temperature of 40 ํC was, however, unfavorable for the process. The TKN removal efficiencies at the said temperatures were 67, 89, 95, 94 and 57 percent, respectively, i.e., it increased with temperature until a certain level after which (40 ํC) the efficiency drastically dropped. The TN removal efficiencies were 63, 62, 73, 60 and 49 percent, respectively (the TN removal was not the main objective of this study, and the system was not set to have a proper anoxic-denitrification process). In addition, the MLVSS at the 35 ํC scenario was more than that at 15 ํC (2688 VS 1199 mg/l) eventhough the P removal efficiency was lower at the higher temperature, probably because at this 35 ํC, the proportion of the Glycogen Accumulating Organisms (GAOs) and Ordinary Heterotroph Organisms (OHOs) in the bacteria mass increased. In conclusion, the BPR efficiency at low temperatures (5, 15, 25 ํC) was better than at high temperatures (35 and 40 ํC). This means that the EBPR process will work better in the temperate climate than the tropical ones like ours (Thailand).