We report the ductile fracture behavior of a tough Zr-based bulk metallic glass (BMG) in the dynamic regime, where a rapid moving crack initiates by a Mode II shearing and then advances through plastic voids growth and linking. A clear fractographic evolution from river-like vein pattern to chevron-like zone where microscopic dimples and voids coexist, followed by crack microbranching pattern, was observed on the dynamic fracture surfaces. A terminal crack velocity of about 0.5 of the Rayleigh wave velocity was determined for the onset of such microbranching instability. Furthermore, the dynamic branched daughter cracks are favored to continue in their original mother crack plane, having ''directional stability''. Our findings may increase the understanding of the dynamic fracture of BMGs with both glassy nature and metallic bonding character. (C) 2011 Elsevier Ltd. All rights reserved.