Traditional Mo-Ni fillers for high temperature brazing of refractory materials have been used in various applications for a long time. However, properties of these fillers have never been sufficiently studied. Their properties, such as the melting point and a preferred temperature history, background atmosphere, flowability through capillary gaps, as well as mechanical properties play a key role in a successful high temperature brazing process and advanced product quality. In this study Mo-Ni eutectic powder fillers have been doped with either (i) 3 wt% and 100 wt% Mo-Ni nanoparticles (<100 nm), or (ii) 1 wt%, 3 wt% and 5wt% SiC nanoparticles (<30 nm), thus forming more or less homogeneous or heterogeneous nano-composites, respectively. These soformed fillers have been used, in this study, for brazing of (i) Molybdenum to porous Tungsten and (ii) Molybdenum to Molybdenum, respectively. Mechanical properties of resolidified fillers were studied in comparison with non-doped fillers by using continuous instrumented microindentation, and conventional hardness testing. It is noticed that, compared to non-doped fillers, (i) fillers with 3 wt% and 100 wt% Mo-Ni nanoparticles feature a slightly increased hardness with a decreased penetration depth; and (ii) fillers with increasing amount of SiC nanoparticles feature a significant increase in hardness, by up to ∼130%. Moreover, the marked differences in mechanical responses of various phases within the joint domain were identified.