A wet-chemical synthesis process was designed to obtain reproducible single-phase multiferroic BiFeO3 nanoparticles. The phase purity, single crystallinity, and size of the nanoparticles are confirmed through the analysis of Xray diffraction patterns, Raman spectroscopy, and high resolution transmission electron microscopy experiments. Crystal nucleation happens within the amorphous-rich area in multiple seeds, leading to the formation of single crystalline nanoparticles with no preferential faceting. Crystallization mechanisms of BiFeO3 nanoparticles were investigated following the Kissinger-Akahira-Sunose approach, indicating that two crystallization steps are responsible of the complete BiFeO3 nanoparticle formation. The first crystallization step involves a maximum of 70% of the final crystal volume, arising from nanocrystal nucleation and growth. The second step occurs above this threshold crystal volume fraction, and it is related to the nanocrystallite coalescence process. Analysis of the thermodynamic process of the crystallization of BiFeO3 nanoparticles following Ostwald rules suggests a relatively low energy barrier for crystal nucleation, highlighting that phase pure, single crystalline BiFeO3 nanoparticles are obtained using the present optimized wet-chemical synthesis process, with temperatures as low as 450 degrees C.