Roles of reactive species of germanium and silicon plasma nitridation were investigated by comparing nitrogen plasma chemistry and oxynitride layer physical properties. In high pressure remote plasma nitridation process, hydrogen containing neutral radicals (NH* and H*) were important to nitride germanium and silicon substrates. This process required high substrate temperature to nitride germanium substrate, whereas silicon substrates could be nitrided at low substrate temperature. In low pressure RLSA plasma nitridation process, N2 + ion species acted as dominant reactive species. Using this process, germanium could be nitrided at low substrate temperature without hydrogen and high nitrogen concentration (~22at.%) GeON was obtained.