Nutrients in wastewater enrich the environment and cause environmental problems in aquatic environments. Conventional wastewater treatment systems are not capable of removing phosphorus effectively. The entry of phosphorus into water resources leads to the formation of an Eutrophication phenomenon. Micro-algae have been more effective among the various methods used for the removing of nutrients from wastewater. Wastewater biological-treatment using algae to eliminate nitrogen and phosphor and produce oxygen for aerobic bacteria is a new trend that has drawn a great deal of attention. Using sunlight, microalgae consume nutrients in wastewater and convert them into useful biomasses. Wastewater biological-treatment using microalgae was studied. Treatment efficiency of micro-algae depends on the species, algae mass concentration, aeration level, and the optimal time to maximize algae activity on wastewater.