Abstract Refinement and modification performance of Al–Ti–B master alloy alone or with Al–P master alloy on primary Mg 2Si in Al–12.67% Mg–10.33% Si alloy was investigated in this paper. The experimental results show that perfect effect can be obtained after addition of 1% Al–5Ti–1B master alloy into the Al–Mg–Si alloy. The morphologies of primary Mg 2Si particulates change from dendritic to polygonal shape, and their average sizes decrease from ∼100 to ∼20 μm. The ultimate tensile strength of the alloy increases. EPMA and TEM results show that TiB 2 particles act as the nuclei of primary Mg 2Si. Crystal lattice correspondence indicates TiB 2 has a good lattice matching coherence relationship with Mg 2Si, and the disregistry is only 4.64%. Furthermore, the addition of Al–5Ti–1B master alloy can significantly improve the refinement and modification effect of AlP on primary Mg 2Si. It is also found that coupling particles of AlP and TiB 2 exist in the center of primary Mg 2Si.