For the vapor-phase synthesis of iron particles from FeCl₂ at temperatures ranging from 800 to 950 ℃, the reason is sought why the model based on the classical nucleation theory brought an increase of particle size with temperature increase, in reverse to experimental observation. The nucleation rate according to the classical theory should decrease with a temperature increase, due to the decrease of super-saturation ratio resulting from the increase of vapor pressure. The decrease of nucleation rate ultimately leads to an increase of particle size. Yang and Qiu''s nucleation theory was applied in place of the classical theory. However, the same result as with the classical theory was obtained : the nucleation rate decreased with the temperature increase. Finally, an Arrhenius-type nucleation rate equation was introduced. The preexponential factor and the activation energy for nucleation were determined to be 1348.2 sec^-1 and 159.1 KJ/mol, respectively. With these values put into Park et al.''s model, good agreement was obtained in temperature dependence of particle size between model prediction and experimental data.