This paper reviews the important improvements accomplished during the past thirty years in the application field of the diamond anvil cell to high pressure single crystal X-ray structure analysis . The characteristics of the single crystal high pressure method are reviewed in Sec. 2. The implications of the single crystal structural analysis at high pressure are discussed for the case of structural anomalies of forsterite, Mg2SiO4 and fayalite, Fe2SiO4 under metastable condition with over-pressure inSec. 3.
