By using Nb and Si elemental powders as raw materials, dense Nb/Nb5Si3 composites were successfully fabricated by a spark plasma sintering (SPS) technology. The microstructure of the fabricated composites was analyzed by OM, SEM, XRD and EPMA; the microstructure evolution of the composites was also investigated by a quenching test. The experimental results show that the prepared composites consist of Nb and Nb5Si3 phases; Nb particle uniformly distributes in the in-situ synthesized Nb5Si3 matrix. During the SPS process, an interfacial reaction occurs between Nb and Si particles to synthesize Nb5Si3 until reactant silicon has been completely depleted.