The effects of phosphorus on primary, eutectic Mg 2Si phases and growth manner of Mg 2Si crystal in Mg 2Si/Al composite with steel die and sand mould cast are investigated. The results show that, with the addition of phosphorus, the morphologies of primary Mg 2Si particulates changed from dendritic or equiaxed to polygonal shape, and their sizes decrease from ∼300 μm and 200 μm to ∼20 μm and ∼50 μm, respectively. Furthermore, the morphologies of eutectic Mg 2Si phases change from fibriform to short fibriform and/or dot-like, while the eutectic Si phases do not change obviously. The exact morphologies of Mg 2Si crystals are identified as tetrakaidecahedron and/or octahedron after modification with P, due to V [1 0 0] is depressed. Also, growth mechanism and the formation of hopper-like crystal are discussed.