The mechanical properties of Ni-19Si-3Nb-xB alloys were investigated by microhardness and tensile tests. X-ray diffraction, high-temperature differential thermal analysis, and scanning electron microscopy were conducted to characterize the microstructure change by the additions of boron for these alloys. The results reveal that there are three phases, α(fcc) phase, β(L12) phase, and the Nb-rich precipitates in the as-cast Ni-19Si-3Nb alloys. The strengthening effect of this alloy is suggested to be due to the lattice expansion of the β phase matrix by the addition of Nb and the hardening by the hard Nb-rich precipitates on the β matrix. Additionally, the addition of boron element does not appeal to affect their phase content and thermal analysis results. But, the fracture behavior of the Ni-19Si-3Nb alloys significantly change from brittle mode to ductile mode with increasing boron doping. The relationship between the effect of boron doping, the microstructure change, and the fracture mode on the Ni-19Si-3Nb based alloys are discussed in this paper.