The effects of Si/Al ratio of Al-containing MCM-41 materials prepared by either aluminum incorporation or postsynthesis alumination on the formation or prevention of structural defects and improvement of hydrothermal stability of MCM-41 have been investigated in detail. The states of Al and Si in the Al-containing materials are analyzed by MAS NMR, and the detailed properties of the materials subjected to hydrothermal treatment under different conditions are characterized by ICP, XRD, N-2 adsorption, and FTIR. X-RD and N-2 adsorption results indicate that the amount of structural defects formed during calcination increases with the amount of aluminum in Si-Al-MCM-41. After calcination, Si-MCM-41 and Si-Al-MCM-41 having high Si/Al ratio (greater than or equal to50) still show uniform pore structures, but structural defects are formed on the calcined Al-rich MCM-41 (Si/Al = 25 or 10). Structural defects are formed on all Si-Al-MCM-41 materials during hydrothermal treatment in boiling water. Although the mesoporous framework of Si-Al-MCM-41 is still found to be somewhat preserved after treatment in boiling water for 1 month, serious blockage of the pore channel is observed. To prevent the formation of structural defects, postsynthesis alumination has been found to be an effective method to prepare Al-containing MCM-41 with uniform pore structure. Moreover, the resulting uniform mesoporous structure of alumina-modified MCM-41 could be maintained, without the formation of structural defects, upon hydrothermal treatment in boiling water for I week.