In-situ Raman and X-ray diffraction experiments have been performed on mineral natrolite (Na-zeolite) to investigate the dehydration behavior under high temperature and high pressure-temperature conditions. XRD data show reversible dehydration up to 450 ℃ whereas Raman data show irreversible change when the material is heated up to 630 ℃. The existence of post-natrolite was newly identified to form from β-metanatrolite at 380 ℃ upon cooling. The formation of dehydrated metanatrolite under simultaneous high temperature-pressure (room temperature < T < 300 ℃, 0 GPa < P < 2.1 GPa) conditions was not observed.