Al-pillared montmorillonites (Al-MMT) were synthesized from naturally occurring sodium montmorillonite through exchange of interlamellar ions with hydroxyalumina polycations. Furthermore, Powder X-ray diffraction (XRD), Fourier transformation infra-red spectra (FTIR), Thermogravimetric analysis (TG), and differential scanning calorimetry (DSC) were applied in order to study the themal stability, hydrothermal stability and synthesis mechanisms of pillared materials. The Al-MMT are thermally stable and hydrothermal stability up to700°C. X-ray diffractograms of Al-MMT calcined at 700 0C show a sharp and intense d001 peak, corresponding to a basal spacing of 1.66nm. Until calcinations at 8000C, the pillared clays generally retain their structural ordering as evidenced by a broad basal (001) peak, which is an indication that the layer structure lost some long-range ordering. In a 100% steam flow under atmospheric pressure at 7000C, the characteristic d001 spacing of Al-MMT decreases from 1.83nm to 1.71nm, intensity of basal peak reduced by58%. However, after hydrothermal at 8000C, the basal (001) peak is not observed, indicating a complete structural collapse. Synthesis mechanisms for Al-MMT are established. Some of tetrahedral silicons or aluminums inverse towards gallery to react with pillaring agents and yield Alp-O-AlsⅣ or Alp-O-Sis bridges which would firmly fix the pillars to the host clay.