Abstract: The alloys containing Mo-12Si-8.5B and Mo-28Si-8.5B respectively were prepared by the in-situ synthesis reaction of the elemental powders of Mo, Si and B by hot-pressing. The microstructures and the annealing processing were investigated by optical microscopy, polariscopy, scanning election microscopy (SEM), and X-ray diSractometry (XRD). The fracture toughness of the alloys at room temperature were determined to be 7.7 and 4.1 MPa·m^1/2 respectively. The mechanism for toughening at ambient temperature was discussed.