The degradation mechanism of Mo_xW_1-xSi₂ ultrahigh-temperature heating elements fabricated by selfpropagating high-temperature synthesiswas investigated. The Mo_xW_1-xSi₂ specimens (with and without post-annealing) were subjected to ADTs (accelerated degradation tests) at temperatures up to 1,700℃ at heating rates of 3, 4, 5, 7, and 14℃/min. The surface loads of all the specimen heaters were increased with the increase in the target temperature. For the Mo_xW_1-xSi₂ specimens without annealing, many pores and secondary-phase particles were observed in the microstructure; the surface load increased to 23.9 W/㎠ at 1,700℃, while the bending strength drastically reduced to 242 MPa. In contrast, the Mo_xW_1-xSi₂ specimens after post-annealing retained single- Mo_xW_1-xSi₂ phases and showed superior durability after the ADT. Consequently, it is thought that the formation of microcracks and coarse secondary phases during the ADT are the main causes for the degraded performance of the Mo_xW_1-xSi₂ heating elements without post-annealing.