Aluminum nitride (AlN) is one of the attractive ceramics applicable to the surface modification because of its excellent properties in chemical stability and thermal conductivity. In this research AlN coating was fabricated by reactive RF (Radio Frequency) plasma spray process, a kind of thermal spraying techniques. Reactive plasma spraying, in which metal element reacts with surrounding active species in plasma, has been considered to be an useful process for the formation of non-oxide ceramics thick coatings. By increasing nitrogen content in plasma gas, AlN coating without pure Al part was attained while the coating microstructure was heterogeneous, brittle and quite porous. By decreasing nitrogen content in plasma gas, on the other hand, Al/AlN composite coating with more homogeneous, less porous microstructure could be attained. Changing nitrogen fraction in plasma gas may be effective for controlling AlN content in Al/AlN composite coating. Nitriding process of aluminum in reactive RF plasma spraying was also investigated in this study. It could be considered that nitridation process of Al was occured during the particle flight in plasma or after the particle deposited onto the substrate. Nitriding reaction process of Al in the reactive plasma spray process was verified in the study.