Nb/Nb5Si3 in-situ composites are very attractive structural materials for these materials perform a good balance in mechanical properties, including a high strength at high temperature (>1373K) and reasonably high fracture toughness at room temperature. Metastable phase Nb3Si plays an important role in the properties of Nb/Nb5Si3 composites by affecting microstructure and volume fracture of ductile phase. In this paper, Nb-10Si-xMo and Nb-18Si-xMo (x=0,5,15) are prepared by arc melting and annealed at 1473K for 100h. Single edge-notched bending (SENB) test was used to study the fracture toughness of Nb-Si-Mo alloys. The stability of metastable phase is analyzed by XRD. The room temperature fracture toughness of Nb-10Si is 10.47MPa(m)1/2 and higher than that of binary Nb-18Si alloys at near-eutectic compositions. The addition of Mo improves the fracture toughness of as cast Nb-Si alloys from 4.1 MPa(m)1/2 to 9.9MPa(m)1/2 at near-eutectic compositions and decreases it from 10.47 MPa(m)1/2 to 8.8MPa(m)1/2 at hypoeutectic compositions.