Abstract: For deuterium clusters and methane clusters produced by specific jet nozzle, Rayleigh scattering has been employed to investigate the cluster formation process and to measure the cluster size at different gas back pressure. The comparison between Hegena’s theory and our experiment shows that, experimental sizes of deuterium and methane clusters are larger than their respective theoretical ones; the methane clusters derived at 298 K is smaller than the deuterium clusters derived at 80 K, but hydrogen atoms in methane clusters are at least 1.98 times the number of deuterium atoms in deuterium clusters under the same back pressure.