The effect of particle size range on oxidation behavior was investigated according to exposure time in isothermal oxidation condition. Emphasis was placed upon oxygen content, porosity, and oxide scale formation. Commercially available CoNi- and CoCrAlY powders of several different particle size ranges were vacuum-plasma sprayed on a nickel alloy substrate. The results show that the isothermal degradation of coatings is considerably influenced by the particle size distribution. It can be clearly observed that a remarkable increase in the oxygen content in the as-sprayed coating occurred with a decrease in the mean particle size. But after thermal exposure, the difference of the oxygen contents between the smaller and larger particle coatings is decreased. The distribution of particle size plays the important role of porosity than only the mean particle size. The powder which has the widest range and sample variance leads to make good porosity inside coatings during the deposition process.