The roles of nucleation temperature and heat transfer of constituents used in the synthesis processes of transition metal boride (TMB) nanoparticles by radio frequency (RF) thermal plasma were investigated through the comparison between titanium and iron boride nanoparticles prepared in different raw powder feed rates. The molar ratio of metal boride in the product was decreased with increasing powder feed rate both in Ti-B and Fe-B systems. The main reason is extended evaporation time and vapor distribution of boron at high powder feed rate, which results in less reaction between boron nuclei and metal vapor. Meanwhile, titanium boride had the smaller mean particle diameter of 24.0 nm compared with that of 34.5 nm for iron boride. The size distributions of nanoparticles in Ti-B and Fe-B systems are 15-35 nm and 25-55 nm, respectively. In quantitative analysis, the content of titanium boride nanoparticles in the product was larger than that of iron boride. In both Ti-B and Fe-B systems, boron is nucleated first due to the lowest saturation vapor pressure, and then metal and boron vapors co-condense on boron nuclei. Since the melting point of boron is higher than nucleation temperatures of titanium and iron, high nucleation temperature of titanium leads to relatively long co-condensation time of boron and titanium in liquid state and relatively short co-condensation time of boron and titanium in supercooled liquid state compared with iron case. (C) 2013 Elsevier B.V. All rights reserved.