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AZ61 alloys with different levels of Al5Ti1B master alloy additions were prepared by conventional casting method. The effects of Al5Ti1B contents and holding time on microstructures and microhardness of AZ61 alloys were studied by XRD, OM and microhardness testing techniques. The results show that when the addition level of Al5Ti1B master alloy is less than 0.5% (mass fraction), the average grain size of the alloys decreases with the increase of Al5Ti1B content at the same holding time. But the grain size increases somewhat with further addition of Al5Ti1B. The average grain size of the alloys decreases with the increase of the holding time as it is less than 30 min at the same addition level of Al5Ti1B. It is considered that TiB2 particles can serve as the heterogeneous nucleation sites of α-Mg during solidification, and heterogeneous nucleation is the main reason for the grain refinement of AZ61 alloys. The microhardness of the refined AZ61 alloys with 1.0% Al5Ti1B addition is increased by about 8%.