We report experimental evidence of in-situ Mg 77 Cu 12 Zn 5 Y 6 bulk metallic glass (BMG) matrix composite with extraordinary plastic strain of 18.5% and specific strength of 4.31×10 5 N·m·kg −1 , which are the highest plasticity and specific strength in Mg base BMG alloys reported to date. The excellent mechanical properties are attributed to the formation of the composite structure which is composed of amorphous matrix and hexagonal-close-packed (hcp) Mg solid solution needle phase with a width less than 500 nm. As plastic phase, the Mg needles not only possess the capability of deformation but also have work hardening phenomenon effect. They give rise to multiple shear bands, and hinder the propagation of local shear band in amorphous matrix.Keywords: Mg 77 Cu 12 Zn 5 Y 6 , bulk metallic glasses, in-situ composite, plasticity, specific strength.Mg base BMG alloys have attracted a great deal of attention for its low density and relative low cost. To date, BMG alloys with millimeter scale have been fabricated by using conventional copper cast and die cast method in Mg-Ni-Ce [1] , Mg-Ni-La [2] , Mg-Cu-Y [3] , Mg-Ni-Y [3,4] , Mg-Ni-Nd [5] , Mg-Cu-Y-Al [6] and Mg-CuAg-Y [7] systems. Mg 65 Cu 20 Zn 5 Y 10 [8] and Mg 65 Cu 15 Ag 5 -Pd 5 Y 10 [9] BMG rods with the diameter of centimeter scale have been also successfully developed. In comparison with Mg base crystalline materials, Mg base BMG alloys exhibit great improvement in strength, e.g. the compressive strength of Mg 65 Cu 15 Ag 5 Pd 5 Y 10 [9] and Mg 65 Cu 20 Zn 5 Y 10 BMG alloy is 650MPa and 800 MPa [8] , respectively, which is more than 2 times that of crystalline Mg alloys. Recently, Xi et al. [10,11] reported that the addition of rare earth elements (RE = Gd, Pr, Nd, Tb, Y and Dy) can significantly improve the resistance to the deterioration of glass forming ability and manufacturability of Mg-based alloys caused by oxygen in the environments. Mg 65 Cu 25 RE 10 (RE = Gd, Tb and Dy) BMG rods with diameter of 5 mm is easily prepared under the atmosphere of 1×10 −2 Pa. Due to the lack of dislocation sliding mechanism as in crystalline materials, bulk metallic glasses fail to form intense shear bands that can propagate catastrophically. Local adiabatic heating and softening phenomena are usually produced during deformation. Most BMG alloys fracture along the 45° plane to stress axis when loading stress exceeds their elastic ultimate strength. Therefore, BMG alloys usually have poor compressive plasticity. For Mg base BMG alloys, the fracture characterization of shear location is more obvious than other BMG alloys. Many Mg base BMG alloys even fail in the elastic regime. In order to overcome this drawback, Xu et al. [12,13] prepared TiB 2 particulate reinforced Mg 65 Cu 20 Zn 5 Y 10 and Mg 65 Cu 7.5 -Ni 7.5 Zn 5 Ag 5 Y 10 BMG matrix composite. These composites exhibit not only improved strength but also obvious compressive plasticity. For example, the compressive strength and the plasticity of the Mg 65 Cu 7.5 -Ni 7.5 Zn 5 Ag 5 Y 10 BMG matrix composite con...