Thermodynamic characteristics in a diffusion chamber are investigated theoretically. A metallic gas evaporated at the center of a chamber is assumed to diffuse through an inert gas without condensation until it arrives at the walls. The supersaturation in the chamber is computed under various values of the temperature of the vapor source and the total pressure in the chamber. Under the assumption that the supersaturation determines the size of the metallic fine particles comparison is made between the results of the present theory and those of recent experiments.