The microstructures and mechanical behavior of the as-cast and isothermally annealed Zr63Al9.7Ni9.7Cu14.6Nb3 bulk metallic glasses (BMGs) were studied by differential scanning calorimetry (DSC), X-ray diffraction (XRD), transmission electron microscopy (TEM), and room temperature uniaxial compression. The as-cast BMG alloy shows a wide undercooled liquid span of 73 K at a constant heating rate of 40 K/min. Composite microstructures containing nanometer scaled icosahedral quasicrystals (i-phase) were produced upon annealing at 705 K. Under uniaxial room-temperature compression at a strain rate of 510-4 s-1, the as-cast BMG alloy exhibits a elastic deformation εy ~ 1.95%, a yield stress σy ~ 1650 MPa, and a Young’s modulus E ~ 84.5 GPa. The alloy shows a plastic strain εp ~ 8.0 % in a serrated plastic deformation process. Annealing induced embrittlement was observed in the relaxed BMG alloys. Comparing with the as-cast alloy, the relaxed and the composite alloys show negligible changes in elastic strain and Young’s modulus. The partially crystallized alloys are macroscopically brittle. Well developed vein patterns were observed in the fracture surfaces of all these alloys. The present work revealed that the dispersion of nanometer scaled i-phase particles is not effective as a barrier against shear localization in these partially quasicrystallized alloys.