Ternary alloying of Mo5Si3 with adding a series of transition element was investigated by x-ray diffraction (XRD), optical light microscopy, scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The EDS results showed that Zr, Ti, Co and V have certain solubility in homogenized Mo5Si3, which were determined to be (2.20±1.42)%,(15.94±0.18)%,(3.33±0.76)% and (7.43±0.22)%, respectively. Microstructural characterization indicated that all studied alloys have a two phase microstructure, i.e., Mo5Si3 matrix and the second phase Mo37Zr20Si43, Mo66Si19Ti15, MoCoSi and (Mo, V)3Si. The fracture toughness Kc was determined by indentation technique. The result showed that alloying with above transition elements led to a little change in fracture toughness.