Nb/Nb5Si3 in-situ composites are very attractive structural materials because these materials perform a good balance in mechanical properties, including high strength at high temperature (＞ 1000℃) and reasonably high fracture toughness at room temperature. Metastable phase Nb3Si plays an important role in the properties of Nb/Nb5Si3 composites by affecting the volume fraction of ductile phase. The addition of Mo affects on the microstructure of ductile phase and the stability of metastable phase Nb3Si. In this paper, Nb-10Si-xMo and Nb-18Si-xMo (x = 0, 5, and 15) alloys were prepared by arc melting and annealed at 1473 K for 100 h. Single edge-notched bending (SENB) test was used to study the fracture toughness of Nb-Si-Mo alloys. The room temperature fracture toughness of Nb- 10Si is 10.48 MPa. m1/2 and higher than that of binary Nb- 18Si alloys at near-eutectic composites. The addition of 5 at.% Mo improved the fracture toughness of as-cast MPa. m1/2 at hypoeutectic compositions.