The Ni-19Si-3Nb alloy was selected as the base alloy and microalloyed with an increasing amount of carbon and boron to improve its mechanical properties. The specimens were prepared by are melting and drop casting under argon atmospheres, annealing in a vacuum, and then machining for mechanical testing. The evolution of microstructure change and mechanical properties of the Ni-19Si-3Nb-xB-yC alloys were characterized by X-ray diffraction, DTA, SEM, microhardness tests, and tensile tests. The results of X-ray diffraction and DTA could not determine any change of phase content for the Ni-19Si-3Nb alloy with varied carbon and boron content. However, the consequences of the fracture behavior of the Ni-19Si-3Nb alloys with different carbon and boron additions exhibit significant change from brittle mode to ductile mode. The optimum additions of carbon and boron for improving the mechanical properties of the based alloy occur at the combination of 200-p.p.m. carbon and 300 p.p.m. boron addition. The relation between the effect of carbon and boron addition, the microstructures change, and the mechanical behaviors on these Ni-19Si-3Nb-xC alloys are discussed in this paper.