Novel metal nanoparticles of Pd and Pt were prepared by sonochemical reduction of solutions containing H₂PtCl₆ or K₂PdCl₄. The effect of atmospheric gas on the particle size distribution was investigated. Average diameters and standard deviations of the Pd particles prepared under Ar (Pd/Ar) and N2 (Pd/N₂) were found to be 3.6 ± 0.7 nm (Pd/Ar) and 2.0 ± 0.3 nm (Pd/N₂). Smaller and sharper distribution of the particle size was observed for the Pd particles formed under a N₂ atmosphere. In the case of Pt, a smaller and sharper distribution of the particle size was observed for the particles prepared under a Xe atmosphere. These relations can be explained in terms of a hot-spot temperature created by acoustic cavitation.