The A₂O process, which is used to treat most of the wastewater generated in Korea, induces the luxury uptake of phosphorus-accumulating organisms (PAO) through anaerobic-aerobic conversion. Moreover, it can remove phosphorus by wasting microorganisms containing large amounts of phosphorus. However, when the nitrate in the return sludge enters the anaerobic tank, the diminished reducing power of the anaerobic tank inhibits phosphorus release, which limits the phosphorus removal efficiency. In this study, the possibility of enhanced phosphorus removal is explored under the anaerobic-aerobic method by using the cyanobacterium Synechococcus sp., which can synthesize polyhydroxybutyrate (PHB) like PAO. If Synechococcus sp. can remove excess phosphorus according to the same mechanism as PAO, it can possibly have a synergistic effect on phosphorus removal through photosynthesis metabolism as well as luxury-uptake due to anaerobic-aerobic conversion. The study results indicate that there was no significant difference in growth and carbon and phosphorus removal efficiencies, regardless of exposure to anaerobic conditions. Therefore, it the anaerobic condition can be considered to not inhibit the growth and metabolism of Synechococcus sp. Under anaerobic conditions, Synechococcus sp. released 28 mg-P/g-MLSS/day of phosphorus, which is lower than the amount released by PAO bacteria. There was no significant difference in the amounts of phosphorus removal between the anaerobic-aerobic conversion experiment and the control. In addition, the trend of carbon consumption was not consistent with the trend of phosphorus release. Therefore, at the very least, phosphorus release by Synechococcus sp. might follow a different pathway, or Synechococcus sp. does not release phosphorus, unlike PAO. Therefore, according to the results of this study, the application of Synechococcus sp. for phosphorus removal, similarly to PAO bacteria, through the anaerobic-aerobic conversion method is inappropriate.