Abstract: CaO-MgO-SiO₂ ceramic fibers were prepared by the high-temperature melting method, and then heat-treated at temperatures of 800-1 200℃. The effects of the heat-treatment temperature on the surface morphology and on the hydration resistance of the ceramic fibers were studied. The results show that the crystallization of the fiber changed from the surface crystallization to the overall crystallization due to the increasing heat-treatment temperature, and the fibers showed a coexisting structure of crystalline state and amorphous state. The structure difference between the different temperature heat-treated fibers and the existence of the phase boundary resulted in the different reaction activities between fibers and OH^- during the soaking process in the deionized water, furthermore significantly affected the hydration resistance of the heat-treated fibers.