Zr41.25Ti13.75Ni10CU12.5Be22.5 (at.%) bulk metallic glass (BMG) is annealed at 676 K for various times. The influence of the microstructural evolution on the mechanical properties is studied by tensile test and micro hardness test at room temperature. The structural relaxation due to the short time annealing does not affect the deformation behavior of the BMG. The nanocrystallization inducing embrittlement is due to the presence of nano-scale particles promoting the crack forming and propagating during the tensile deformation. The existence of nano-scale particles also enhances the viscosity of the amorphous phase within the shear band and even obstructs the formation of the shear band at the indenter tip, which results in a strengthening phenomenon in the annealed BMG under the indentation load. (c) 2005 Springer Science + Business Media, Inc.