This article describes the room-temperature and high-temperature mechanical properties and failure modes of series Nb-W-Si alloys-Nb-10W, Nb-10Si, Nb-10Si-5W, Nb-10W-5Si, and Nb-10W-10Si-prepared by arc melting. For the Nb-10W alloy, the microstructure was a monolithic Nb solid solution (Nb ss ) with a grain size up to a few hundred microns, while the other four alloys consisted of primary Nb ss and a eutectic of Nb ss /Nb 5 Si 3 (5-3 silicide) as a result of replacing Nb with Si. Among all alloys, the Nb-10W showed the highest fracture toughness of about 15.3 MPa 1/2 and the lowest 0.2 pct yield compressive strength of 90 MPa at 1670 K. Conversely, the Nb-10Si-10W had the highest 0.2 pct yield strength of about 330 MPa at 1670 K and the lowest fracture toughness of 8.2 MPa 1/2 . It is suggested that toughness is supplied by the metallic Nb ss phase, while high-temperature strength is mainly provided by the brittle silicide phase. For the Nb-10W alloy with the monolithic Nb ss , intergranular cleavagelike crack propagation is the fracture mode at room temperature, and dislocation movement within the grains and grain-boundary sliding are the dominant modes of high-temperature failure. With two-phase Nb ss /Nb 5 Si 3 microstructures, the compressive damage of all four alloys at high temperature was dominated by debonding of the interfaces between the Nb ss and the silicide; however, the fracture mode at room temperature is transgranular, controlled by the primary Nb ss cleavage. 1m 1m