Recently, Nb-Si based alloys have attracted considerable attention as potential candidate materials for ultra-high temperature applications, because of their low densities and high melting points. However, it is still very difficult to obtain materials with a good balance of high-temperature strength and room-temperature toughness. To address this issue, microstructure control is considered to be a promising method. In applying microstructure control to Nb-Si based alloys with a eutectic reaction (L → Nbss + Nb3Si) and a eutectoid reaction (Nb3Si → Nbss + Nb5Si3), the key is the control of Nb3Si phase stability. Nbss (Nb solid solution) is considered as a ductile phase. In previous reports, it was revealed that different elements had different effects on the stability of Nb3Si. In particular, Mo and W (>3 at %) destabilize the Nb3Si phase, while Ti and Ta stabilize it, and Zr acts as an accelerator for decomposition of Nb3Si. On the other hand, Cr is known to enhance the formation of the ductile Nbss phase. In the present study, we investigated the effects of adding combinations of stabilizing, destabilizing, and accelerating elements with Cr, such as Cr and W, Cr and Ta, Cr and Zr. According to SEM observation, different microstructures were obtained with different combination of additives, and the fracture toughness at room temperature of these samples were also evaluated to reveal the effects of the microstructure on the mechanical properties of Nb-Si based alloys.