This short paper reports the direct observation of the structure variation of mesoporous silica at temperatures higher than 600 oC by use of an in situ XRD technique. The mesostructure of SBA-15 or other mesoporous materials such as MCM-41 became almost invisible when the temperature rose to above 600 oC, but recovered or partially recovered once the temperature decreased. Contrarily, the characteristic XRD patterns of zeolites such as ZSM-5 kept unchangeable under the same conditions. On the basis of comparative experiments performed on various mesoporous samples, it is inferred that the reversible variation of XRD patterns probably originates from the thermal shock of the pore wall, not from the permanent collapse of the mesoscopic structure in these samples. This observation indicates the special features of SBA-15 at high temperature.