Metal and ceramic composite materials have been intensively used for manufacturing engineering components. Performances of the composite materials are largely dependent on microstructures, phase composition, and phase distribution. In-situ reactive synthesis of composite powder is considered to improve uniformity of components at the particle level while blended feedstock is generally prepared by mechanically milling or liquid base processes. RF thermal plasma is of great versatility in powder preparation because of its high energy density and reactivity. In this study, aluminum micro-powder was fed into argon-nitrogen thermal plasma in order to prepare Al/AlN composite powder. During the process, in-flight particles interact with thermal plasma individually. Physical phase transformation results in spheroidization and chemical reaction between molten Al and nitrogen results in the nitriding of in-flight Al molten droplet. Characteristics of feedstock Al powder and synthesized composite powder were conducted.