Bulk nanocrystalline Mg2Si(n-Mg2Si) was prepared by mechanical milling and reactive hot-pressing in vacuum and its mechanical properties were characterized. The results indicate that the grain sizes of Mg and Si decrease non-monotonously with increasing ball-to-powder mass ratio(BPR). The content of Mg2Si phase increases with increasing milling time; however, pure Mg2Si phase cannot be obtained by direct ball milling. Moreover, monolithic is Mg2Si phase can only be obtained when as much as 10% of excess Mg was blended in the raw mixtures. The relative density (D) and fracture toughness as well as hardness of bulk Mg2Si samples can be raised effectively by increasing the pressure in hot-pressing. After hot-pressing at 400 ℃ for 60 min under 1.5 GPa, highly dense(D>98%) n-Mg2Si (d=60 nm) was obtained, whose fracture toughness (KIC≈1.46 MPa·m1/2) and Vickers hardness (HV≈ 4.38 Gpa) are superior to those of conventional coarse-grained Mg2Si. Bulk nanocrystalline Mg2Si (n-Mg2Si) was prepared by mechanical milling and reactive hot-pressing in vacuum and its mechanical properties were characterized. The results that that the grain sizes of Mg and Si decrease non-monotonously with increasing ball-to- ratio (BPR). The content of Mg2Si phase increases with increasing milling time; however, pure Mg2Si phase can not be obtained by direct ball milling. Moreover, monolithic is Mg2Si phase can only be obtained when as much as 10% of excess Mg was blended in the raw mixtures. The relative density (D) and fracture toughness as well as hardness of bulk Mg2Si samples can be raised effectively by increasing the pressure in hot-pressing. After hot-pressing at 400 ° C for 60 min under 1.5 GPa, highly The fracture toughness (KIC≈1.46 MPa · m1 / 2) and Vickers hardness (HV≈4.38 Gpa) were superior to those of the conventional coarse-grained (D = 98%) n-Mg2Si Mg2Si.