In this paper, the crystallographic orientation relationship between TiB2 particles and α-Mg phase is identified by TEM to clarify the grain refinement mechanism of TiB2 particulate reinforced AZ91 composites. The influence of the content of TiB2 particles on the grain size and mechanical properties of TiB2/AZ91 composites is investigated. The experimental result reveals that the TiB2 particles can refine α-Mg grains in the AZ91 matrix, and with TiB2 particles increased, the size of α-Mg grains in the composite decreases significantly. There exists a well-defined crystallographic orientation relationship between TiB2 and α-Mg matrix, which is (0001)TiB2∕∕(101¯1)α−Mg, 1¯21¯0TiB2∕∕1¯21¯0α−Mg in the present study. In the meanwhile, the lattice disregistry along the specific orientation relationship is 8.30% based on the theoretical calculation, confirming that TiB2 particles can act as the effective heterogeneous nucleation site of α-Mg grains in TiB2/AZ91 composites. Furthermore, the Brinell hardness, ultimate tensile strength and elongation of the composite are increased simultaneously with the increase of TiB2 particles, which are attributed to the uniform TiB2 particles dispersion, grain refinement strengthening and particles strengthening. •α-Mg grains size decreases with increasing TiB2 particles.•The OR between TiB2 particles and α-Mg phase is identified experimentally.•Nucleation mechanism of TiB2/AZ91 composites is clarified.•The hardness, strength and elongation of composites are improved simultaneously.