This study was carried out to invesigate the removal efficiency of organics, nitrogen and phosphorus with operating conditions in SBRs (Sequencing Batch Reactors). The SBR systems were operated with fill, anaerobic, aerobic, anoxic, settling and draw periods for a cycle time of 24 hr. EM(Effective Microorganisms) was added at the beginning of the anaerobic period. From the experimental results, the optimum cycle time for biological nutrient removal was 6 hr-anaerobic, 12 hr-aerobic, 3 hr-anoxic in modified anaerobic-aerobic process.