The structural phase transition and dehydration for gypsum were investigated by Raman spectroscopy and electrical conductivity up to similar to 12.2 GPa and similar to 723 K. A similar to 5.8 GPa of phase transition was verified at atmospheric temperature. The variations in the sulfate and hydroxyl Raman shifts and a change in electrical conductivity marked the onset of dehydration. At similar to 2.5 GPa, gypsum transformed into bassanite at similar to 523 K, and then completely dehydrated to c-anhydrite at similar to 673 K. And furthermore, the gypsumbassanite and bassanite-anhydrite dehydration boundaries were obtained: P (GPa) = -23.708 + 0.050 T (K) and P (GPa) = -4.907 + 0.011 T (K), respectively.