An in-situ AlN/Mg-9Al composite was synthesized by bubbling nitrogen gas. Nano AlN reinforcement phase was in situ formed and uniformly distributed. AlN as a desirable reinforcement phase can be formed stably in the Mg-9Al alloys. Higher ultimate tensile strengths (190–226 MPa) were obtained in the as-cast alloys. Higher fracture elongations (16–47%) were obtained in the as-cast alloys.