Se0.5Te0.5 semiconductor core optical fibers with phosphate glass cladding were fabricated by using a reactive molten core approach. The core was found to be amorphous with some oxygen diffusing in from the cladding glass. Crystalline Se0.5Te0.5 (c-Se0.5Te0.5) semiconductor core fibers were obtained by a post-drawing annealing process. Significant disparities in conductivity under stress and without stress were observed in c-Se0.5Te0.5 semiconductor core fibers. To the best of our knowledge, this is the first time that stress sensing can be realized in c-Se0.5Te0.5 semiconductor core optical fibers. This is in contrast to all previous work on stress sensing using fibers, which require the use of optical probing signals. In addition, it is found that more than 60 times change in conductivity between dark and illuminated states in c-Se0.5Te0.5 semiconductor core fibers. These results demonstrate that c-Se0.5Te0.5 semiconductor core optical fibers have promising applications in stress sensor, photodetector, and optical switch.