In this study, the effect of the thickness of a silica-based ceramic core on its creep deformation, either in air or under vacuum, was studied. Specimens with different thicknesses after sintering were heated to 1550 °C and by maintaining the same temperature for 40, 70 or 100 min. Scanning electronic microscopy (SEM) was employed to observe the surface microstructures and fracture morphologies of the samples. X-ray diffraction (XRD) was used to characterize the phases that formed after creep deformation. The 3D distribution of micropores and the porosities of the specimens at different states were tested by X-ray computerized tomography (XCT). The experimental results show that the effect of the thickness of the specimen on the creep deformation was notably different under different atmospheres. The creep deformation of samples with the same thicknesses under vacuum atmosphere was clearly higher than the deformation in air.In air, the creep resistance was slightly affected by the thickness of the specimen. However, the creep deformation was significantly affected by the thickness of the specimen under vacuum. The creep resistance of the 4-mm-thick specimen is noticeably higher than that of the 0.5-mm-thick specimens. The creep rate of the 0.5-mm-thick specimen is linear with the porosity of the surface layer and higher compared to the 4-mm-thick specimen, which exhibited non-linear behaviour. Reactive micropores were formed on the surface layer of these specimens under vacuum. The volume fraction of the micropores increased with increasing creep time, which resulted in the different characterization of creep for specimens with different thicknesses under vacuum.