The amorphization of cristobalite in a partially crystallized silica fiber (CSF) in the temperature range of 1050 degrees-1350 degrees C in a vacuum was reported previously, in which hydrogen was suggested to accelerate the amorphization process by reacting with the interstitial oxygen, which originated from the heat treatment of vitreous silica fiber in air and then diffused outward in the state of water. In this study, the mass releasing effect and composition change of the CSF heat treated in a vacuum were experimentally identified using thermal gravimetry, differential scanning calorimetry, and quadrupole mass spectrometry. In order to clarify the role of oxygen and hydrogen in the amorphization of cristobalite, the influences of the heat-treatment time, diffusion cross section, temperature, and ambient reductive 5% H-2-95% Ar atmosphere on the amorphization behavior of cristobalite in a silica fiber and silica powder with different initial cristobalite contents and hydrogen concentration were investigated. The phase composition and structure change were characterized by X-ray diffraction and Fourier transform infrared spectrum, respectively. The results confirmed the previous assumption and showed that the amorphization of cristobalite in a moderate temperature range is a reaction- and diffusion-controlled dynamic process.