Thermal evolution of amorphous calcium phosphate (ACP) powder from a fast nitrate synthesis with a Ca/P ratio of 1:1 were studied in the range of 20-980 °C. The powder consisted of amorphous dicalcium phosphate anhydrate (CaHPO4) after heating to 200 °C. CaHPO4 gradually condensed to amorphous calcium pyrophosphate Ca2P2O7 (CPP) between 200 to 620 °C. Amorphous CPP crystallized at 620-740 °C to a metastable polymorph α'-CPP of the high-temperature phase α-CPP and β-CPP. The α'-CPP/ β-CPP phase ratio reached a maximum at 800 °C (60 wt% α'-CPP/40 wt% β-CPP), and α'-CPP gradually transformed to β-CPP at a higher temperature. Some β-TCP occurred at 900 °C, so that a three-phasic mixture was obtained in the powder heated to 980 °C. The occurrence of metastable α'-CPP is attributed to Ostwald's step rule, and a mechanism for β-TCP formation is proposed. The advantages of prospective biomaterials from these powders are discussed.