X-ray amorphous tricalcium-phosphate nanoparticles (ATCP) produced by flame spray synthesis were heat-treated at temperatures between 500 and 1000 °C and analyzed in situ by X-ray powder diffraction. The main phase occurring after crystallisation at 525 °C was α-TCP, minor phases were identified as β-TCP and hydroxyapatite. More elevated temperatures induced crystallite growth and the transformation of α-TCP into β-TCP. Above 900 °C no α-TCP was traceable anymore. α’-TCP was not observed in the experiment. This study shows that nanoparticulate α-TCP can be obtained by thermal treatment of an amorphous TCP nanoparticle in a temperature range where sintering effects such as particle growth and densification are moderate or nearly negligible.