The evolution of the microstructures in Zr-based in-situ bulk metallic glass matrix composites was investigated by semi-solid processing and mechanical testing. A linear relationship between the cube root holding time and the length scales of the precipitated β-Zr phase is established. Moreover, we have demonstrated that the mechanical properties of the composites are closely related to the length scales of the precipitated β-Zr phase. With the increase in the characteristic size scale, the plasticity of the BMG matrix composites is enhanced significantly. When the characteristic size scale of the β-Zr phase exceeds a critical value, this increasing trend becomes very moderate. ; Department of Mechanical Engineering