Magnesium　ammonium　phosphate　(MAP)　crystallization　could　be　utilized　for　the　recovery　of　phosphorus　from　wastewater.　However,　the　effectiveness　of　the　recovery　is　largely　determined　by　the　crystallization　process,　which　is　very　hard　to　be　directly　observed.　As　a　result,　a　specific　ultrasonic　device　was　designed　to　investigate　the　crystallization　characteristics　of　MAP　under　various　ultrasonic　conditions.　The　results　demonstrated　that　the　metastable　zone　width　(MZW)　narrowed　along　with　the　rising　of　the　ultrasonic　power.　Similarly,　for　the　6　mM　MAP　solution,　with　the　ultrasonic　power　gradually　enhanced　from　0　W　to　400　W,　the　induction　time　was　shortened　from　340　s　to　38　s.　Meanwhile,　the　crystallization　rate　was　accelerated　till　the　power　reached　350　W,　and　then　remained　a　constant　value.　It　can　be　observed　from　the　scanning　electron　microscopy　(SEM),　the　MAP　crystal　became　bigger　in　size　as　well　as　the　crystal　size　distribution　(CSD)　became　broad　and　uneven,　with　the　increase　of　ultrasonic　power.　The　results　indicate　that　the　crystallization　process　enhanced　by　power　ultrasound　could　be　used　as　an　effective　method　to　eliminate　and　recover　the　phosphorus　from　wastewater.　(C)　2016　Elsevier　B.V.　All　rights　reserved.