The phase evolution in amorphous calcium phosphate (ACP, with a Ca/P ratio of 1 : 1), derived from the fast nitrate synthesis using different conditions, was studied in temperature range 20-980?C. ACP crystallized within 600-700?C and the phase composition depended on the synthesis duration. It was firstly revealed that for an extremely short synthesis (1min) two metastable phases ??-CPP and ??-TCP of the high-temperature calcium pyrophosphate ?-CPP and tricalcium phosphate ?-TCP were crystallized. For a longer synthesis (5min), ??- CPP and minor ?-CPP crystallized. The metastable phases gradually transformed to stable polymorphs ?-CPP and ?-TCP above 800?C, and a biphasic mixture ?-CPP/?-TCP or ?-CPP formed at 980?C. The crystallization of the metastable phases was attributed to the Ostwald step rule. A mechanism for the formation of TCP (Ca/P = 1.5) from ACP (Ca/P = 1) was proposed. The prepared powders of ?-CPP/?-TCP, ?-CPP or initial ACP were fine-grained and would have enhanced sinterability. Contribution to the densification was demonstrated due to the thermal transformation of the metastable polymorphs into stable phases having higher densities.