As　a　clean,　sustainable　and　efficient　technology　of　wastewater　treatment,　ultrasonic　irradiation　has　gained　special　attention　in　wastewater　treatment.　It　has　been　widely　studied　for　degrading　pollutants　and　enhancing　biological　treatment　processes　for　wastewater　treatment.　This　review　focuses　on　the　mechanism　and　updated　information　of　ultrasonic　technology　to　enhance　biological　treatment　of　wastewater.　The　mechanism　involved　in　improving　biological　treatment　by　ultrasonic　includes:　1)　degradation　of　refractory　substances　and　release　carbon　from　sludges,　2)　promotion　of　mass　transfer　and　change　of　cell　permeability,　3)　facilitation　of　enzymecatalyzed　reactions　and　4)　influence　of　cell　growth.　Based　on　the　above　discussion,　the　effects　of　ultrasound　on　the　enhancement　of　wastewater　biological　treatment　processes　can　be　categorized　into　indirect　and　direct　ways.　The　indirect　effect　of　ultrasonic　waves　in　enhancing　biological　treatment　is　mainly　achieved　through　the　use　of　high　-intensity　ultrasonic　waves.　These　waves　can　be　used　as　a　pretreatment　to　improve　biodegradability　of　the　wastewater.　Moreover,　the　ultrasonic　-treated　sludge　or　its　supernatant　can　serve　as　a　carbon　source　for　the　treatment　system.　Low　-intensity　ultrasound　is　often　employed　to　directly　enhance　the　biological　treatment　of　wastewater.　The　propose　of　this　process　is　to　improve　activated　sludge,　domesticate　polyphosphate　-accumulating　organisms,　ammonia　-oxidizing　bacteria,　and　anammox　bacteria,　and　achieve　speedy　start-up　of　partial　nitrification　and　anammox.　It　has　shown　remarkable　effects　on　maintaining　stable　operation,　tolerating　adverse　conditions　(i.e.,　low　temperature,　low　C/N,　etc.),　resisting　shock　load　(i.e.,　organic　load,　toxic　load,　etc.),　and　collapse　recovery.　These　results　indicate　a　promising　future　for　biological　wastewater　treatment.　Furthermore,　virous　ultrasonic　reactor　designs　were　presented,　and　their　potential　for　engineering　application　was　discussed.