The effect of heat treatment on the microstructure and mechanical properties of an Nbss/Nb5Si3 in situ composites prepared by HRS method was investigated. The microstructure evolution of samples during heat treatment process were analysed by SEM、XRD and EPMA: the result showesd that the as-cast alloy consisted of Nbss、γ-Nb5Si3 、β-Nb5Si3 and primary carbide phase; after heat treatment, the β-(Nb,Ti)5Si3 phase was transformed to α-(Nb,Ti)5Si3 phase and the secondary carbide was precipitated. Meanwhile, the silicide phase gradually dissolved or broken into small pieces, with the morphology changing from the needle into granules. The transformation (Nb,Ti)5Si3 →(Ti,Nb)5Si3 occurred when aging at 1150℃.The compressive strength and room temperature tensile strength of heat-treatment specimens were improved, while the tensile strength of as-cast and heat-treated samples at 1000℃ was at same level. The temperature has a great influence on the fracture mode, cleavage fracture at low temperature and ductile fracture at high temperature.