Several processes for effectively recovering phosphorus have been developed because of its shortage crisis all over the world and serious environmental problems like eutrophication which was attributed to the excess discharge of phosphate compounds. In enhanced biological phosphorus removal processes, activated sludge microorganisms accumulate large quantities of polyphosphate (Poly-P), e. g., under anaerobic-aerobic conditions in wastewater treatment plants. Most of the Poly-P can be released from activated sludge simply by heating it at 70°C for about 1.5h, i. e., 'HEATPHOS' method. Generally, magnesium ammonium phosphate (MAP) which is one of phosphorus recovery materials is crystallized from not Poly-P but orthophosphate (Pi). The purpose of this study is to identify whether the released Poly-P can be crystallized as MAP and to evaluate its forming ability. MgSO<sub>4</sub> and NH<sub>4</sub>Cl were added into the supernatant liquid of activated sludge heated at the above-mentioned conditions. Then the amount of Pi, Poly-P, total phosphorus, ammonium-nitrogen, and magnesium contained in the supernatant liquid was measured. The results were as follows. (1)Approximately 47% of the total phosphorus in the heated supernatant liquid was recovered as MAP at pH9.5. In the case of the liquid containing Poly-P, the recovery rate of phosphorus was higher than 'Pi processes' that MAP is crystallized from Pi. (2)The precipitate which was formed from Poly-P contained less Mg than MAP which is recovered by 'Pi processes'. In conclusion, the phosphorus in activated sludge can be more effectively recovered by crystallizing MAP from Poly-P.