A Ni-19Si-3Nb alloy was selected as the base alloy and microalloyed with various amounts of carbon for improving their mechanical proper-ties. The specimens were prepared by are melting and drop casting under argon atmospheres, annealing in a vacuum, and then machined for mechanical testing. The evolutions of microstructure and mechanical properties of the Ni-19Si-3Nb-xC alloys were characterized by X-ray diffraction, DTA, SEM, microhardness test, and tensile tests. The results of X-ray diffraction and DTA could not resolve any change of phase content for those Ni-19Si-3Nb alloys with varied carbon content. However, the consequences of the fracture behavior of the alloys of Ni-19Si-3Nb with varied carbon addition exhibit significant change from brittle mode to ductile mode. The optimum addition of carbon for improving the mechanical properties of the based Ni-19Si-3Nb alloy is 0.15 at.% (300 ppm.) carbon addition. The effect of carbon addition on the microstructure change and the mechanical behavior of these Ni-19Si-3Nb-xC alloys are discussed in this paper.