Current status on the development of a high pressure and high temperature in situ x-ray diffraction technique is reviewed. Combination of new apparatuses with sintered diamond anvils and a very strong x-ray beam from a synchrotron source made it possible to carry out an in situ x-ray study on mantle minerals under lower mantle conditions 24 GPa, 1900 K). Important information for understanding the nature of the solid earth will be obtained by these apparatuses. © 1994, The Japan Society of High Pressure Science and Technology. All rights reserved.