According to the classical model, one or two iron atoms may be enough to form a critical nucleus in the production of iron powders by reducing ferrous chloride with hydrogen. In the present work, the nucleus size was calculated using the atomistic model and compared with that by the classical model. For a supersaturation ratio of 500,000, the number of iron atoms constituting a critical nucleus was calculated to be 1.32 with the classical model and 48.1 with the atomistic model. The dependency of the nucleus size on the supersaturation ratio was relatively less with the atomistic model. In the calculation of critical nucleus size of materials like iron, the vapor pressure of which is very low, it would be desirable to use the atomistic model, rather than the classical model.