Abstract: The influence of heat treatment on the microstructure and mechanical properties of in-situ nanosized Ti B₂/AZ91 composites was investigated. The results show that the grains of as-cast Ti B₂/AZ91 composites are refined significantly compared with that of AZ91 alloy. During the heat treatment process, Ti B₂particles accelerate the precipitation behavior,resulting in more Mg₁₇Al₁₂phase with smaller size. After heat treatment, the composites exhibit excellent mechanical properties with the hardness, tensile strength and fracture to elongation of 102.5 HBW, 323 MPa and 5.1%, which are increased by 18.4%, 48.2% and 64.5% respectively compared with that of AZ91 alloy. The improvement of mechanical properties is attributed to the grain refinement, mismatch of coefficient of thermal expansion, Orowan strengthening and pinning effect of fine Mg₁₇Al₁₂phase. Tensile fracture morphology shows that there are cracks, tearing edges, dissociation steps and a few dimples observed in AZ91 and the composites, mainly exhibiting brittle fracture, and the improved ductility of the composites is ascribed to grain refinement.