Vacuum diodes are among the most important parts of high electron beam accelerators. In order to make vacuum sealing good, outgassing properties of materials for a ceramic insulated diode is investigated. A pumping model including the influence of flow conductance was set up to simulate the material outgassing rate and pump down time. Finally, the outgassing characteristics of materials between normal temperature and heated state are compared in experiment. Results show that heating can efficiently accelerate the outgassing rate and raise the vacuum sealing time.