High temperature compressive strength at 1773 K and room temperature fracture toughness have been studied in terms of microstructure, phase stability and solid solution hardening in Nb-Si-Mo in situ composites consisting of niobium solid solution and Nb5Si3. Molybdenum addition stabilizes the beta-Nb5Si3 phase and makes unstable Nb3Si phase in the in situ composite. It is found that molybdenum has a strong effect to increase the yield stress of the present in situ composite at 1773 K due to solid solution hardening. Yield strength depends not only on chemical composition and volume fraction but also the Nb5Si3 phase itself. Room temperature fracture toughness is very sensitive to microstructure and the content of ternary alloying element, but not to the volume fraction of constituent phases within the composition ranges investigated. It is suggested that plastic deformation of Nb solid solution and interface decohesion is responsible for high fracture toughness in this alloy system. Details are discussed in relation to microstructural features and Molybdenum alloying.