Employing a diamond-anvil pressure cell and a YAG laser as a heating source, phase relationships in polycrystalline MgF2 (rutile) were investigated to ~330 kbar at both room temperature (~23¿C), and high temperature (~1300¿C). In situ high pressure X-ray diffraction data show that MgF2 (I) of rutile structure transforms into MgF2(II) of a cubic structure at ~300 kbar at room temperature, and at ~190 kbar at high temperature (~1300¿C), resulting in a negative dP/dT slope for the univariant line between MgF2(I) and MgF2(II) in the P-T phase diagram. The molar volume differences at room temperature between MgF2(I) and MgF2(II) at 190 kbar and 300 kbar are ~8% and ~5%, respectively.