The modification effects of Sb on the morphologies of reinforcements Mg2Si and Mg-2(Si,Sn) in Mg-4Si and Mg-5Sn-xSi(x = 1,4) alloys were investigated and the ideal adhesion works (W-ad) between substrate (Mg3Sb2) and crystallization phases (Mg2Si, Mg2Sn and Mg-2(Si,Sn)) were calculated using first principles calculations. The results indicate that the primary alpha-Mg matrix is significantly refined by the modification of Sb. For the eutectic mixture, the arm spacing of the intermetallics of Mg2Si or/and Mg-2(Si,Sn) is decreased. For the primary Mg2Si or/and Mg-2(Si,Sn) which in coarse angular bulk forms without any modification was rounded and distributed uniformly on the alpha-Mg matrix after modification. The primary Mg2Si in octahedrons morphology without modification grows in tetrakaidecahedron mode in Sb modified Mg-4%Si alloys. The clearly straight edges and the sharp corners of the primary Mg2Si become ambiguity, and tend to being spheroidized. Primary Mg2Si, Mg2Sn and Mg-2(Si,Sn) were refined because Mg3Sb2 precipitated during solidification could act as their nuclei. When some Sn atoms replace equivalent Si from Mg2Si phase, the interfaces adhesion work (W-ad) values between substrate and new crystal will increase. Moreover, the interface cohesive strength of Sb-Mg-Si(/Sn/Si-Sn) or Sb-Mg-Mg-Si(/Sn/Si-Sn) will be stronger than the others.