Nb-base in-situ composites with the base composition of Nb-18Si-2HfC were prepared by conventional arc-melting. Their microstructures and mechanical properties, such as high-temperature strength and room temperature fracture toughness, were investigated to elucidate the effects of Re alloying. The in-situ composites predominantly have eutectic microstructures consisting of an Nb solid solution (NbSS) and Nb5Si3. The compressive strength increased with the increasing Re contents at 1470K and not at 1670 K. The strengthening effect observed at 1470 K is higher than that by W and Mo. Re alloying of about 2 % is valuable for improving both the high temperature strength and room temperature fracture toughness of Nb-18Si-2HfC base materials.