Semiconducting Mg2Si films were fabricated on Si (111) substrates by magnetron sputtering and subsequent annealing, and the effects of annealing atmosphere on the Mg2Si film growth were studied. The structural and morphological properties of Mg2Si films were investigated by the means of X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results showed that annealing atmosphere was an important factor that affected the growth of Mg2Si thin films, and vacuum annealing was not suitable for preparing Mg2Si thin films. Only Si (111) substrate diffraction peaks were observed, and no Mg2Si diffraction peak was observed when the first six Mg/Si samples were annealed under vacuum annealing condition. However, many Mg2Si diffraction peaks were observed besides the Si substrate diffraction peaks when the second six Mg/Si samples were annealed under Ar gas atmosphere. In addition, compact and smooth Mg2Si thin films annealed under Ar gas atmosphere were obtained.