Hydrogenated aluminum nitride (AlN:H) thin films have been deposited on silicon (100) wafers by radio frequency reactive magnetron sputtering. Although the as-deposited AlN:H films exhibit smooth surface and low oxygen concentration, they are chemically unstable. The presence of N-H bonds is known to cause chemical instability and an increase of the etching rate in KOH solution. The AlN:H films, which are treated with nitrogen plasma generated by a microwave system, show a much lower etching rate than that of as-deposited AlN:H or even as-deposited AlN films. After nitrogen plasma treatment, the Al-N bonding density in AlN:H films has increased sharply, while N-H bonding density has decreased. It means that the hydrogen atoms in the films diffuse out during the process, so that more chemically-stable AlN:H films are formed.