With Nb and Si powders as starting material, the dense Nb/Nb5Si3 composites were in situ synthesized by spark plasma sintering (SPS) technology. The microstructures of the composites were analyzed through SEM, XRD and EPMA, and the forming mechanism was studied. The results show that the synthesized composites consist of nearly-spherical Nb particles and Nb-Nb5Si3 eutectic. Si powders and surface of Nb particles were melted due to the action of spark plasma during SPS, and the Nb-Nb5Si3 eutectic was formed by eutectic reaction of melted Si and Nb during cooling process. The residual particles of Nb were distributed homogeneously in the eutectic microstructure