Abstract: Annealing at Ta=200℃ or so, the enhancement in the average tensile fracture strength of as-quenched (Fe0.99Mo0.01)78Si9B13 metallic glass increases about 7～8%. The origin of the annealing embrittlement at 244℃ in the metallic glass (Fe0.99 Mo0.01)78Si9B13 was revealed that due to the formation of nanoscale granular with bcc structure resulting in the stress localization.The temperature of annealing embrittlement was in agreement with the calculated value from Tbcc=0.637Tc (Tc is the crystallized temperature), based on micromechanism of crystallization of metallic glass.