The effect of composition and milling time on the mechanochemical behavior of CaO-P2O5-CaF2-ZrO2 system was investigated. Samples were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR) analysis, energy dispersive X-ray spectroscopy (EDX), scanning electron microscopy (SEM), field emission scanning electron microscopy (FE-SEM), and transmission electron microscopy (TEM) techniques. Evaluation of structural features of nanopowders revealed that in the absence and presence of 10 wt.% zirconia the average crystallite size of fluorapatite after 300 min of milling was about 34 and 24 nm, respectively. According to obtained data, by choosing the total milling time to 300 min, the fraction of crystalline phase decreased after 5 min of milling, and then reached a minimum by further increasing milling time to 300 min. On the other hand, it is obvious that with increasing zirconia up to 10 wt.% a similar behavior was observed. The combined FT-IR and EDX results confirmed that the synthesized powders had an appropriate chemical purity. According to SEM/FE-SEM observations, a fine homogeneous microstructure was obtained after 300 min of milling. The average particle size of single phase fluorapatite and its composite structure with 5 wt.% zirconia after 300 min of milling was about 55 and 45 nm, respectively. Based on obtained data by characterization methods as well as thermodynamic values, reaction mechanism steps were proposed to clarify the reactions that happened during the one-step mechanochemical process. (C) 2013 Elsevier B.V. All rights reserved.