A Ni-19Si-3Nb alloy was selected as the base alloy and microalloyed with various amounts of carbon for improving their mechanical properties. The specimens were prepared by arc 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 0.15at.% (300ppm.) carbon addition. The effect of carbon addition on the microstructure change and the mechanical behavior of thes e Ni-19Si-3Nb-xC alloys are discussed in this paper.