A model for submicron particles formation by laser evaporation in ambient gas atmosphere is proposed. Heat transport in the target, evaporation, vapour diffusion in the ambient gas, and kinetics of nucleation and particles growth are considered. The diffusion regime of evaporation is studied. Submicron particles formation by millisecond laser pulses of about 10(4)-10(5) W/cm(2) energy density flux is analysed. The nuclei formation and growth rates are estimated and size distributions of the particles are calculated. The mean particle size decreases with the ambient gas pressure. The spatial separation of nuclei formation and the particles growth processes with a deficiency of condensation centres ensures conditions for a nearly uniform growth which results in narrow particle-size distribution.