Understanding the evolutionary history of the Earth requires studies on the transport properties of constituent materials at high pressures and high temperatures prevailing deep in the Earth's interior. Utilization of intense synchrotron radiation X-rays combined with large-volumes high-pressure apparatus started in the mid 1980's and has brought about new insights on the science of the Earth's interior. Falling sphere viscometry using X-ray radiography enabled us to determine the pressure and temperature dependence of the viscosity of the Fe-FeS melt, which is a candidate for the outer core of the Earth. Studies on phase transition kinetics using in-situ X-ray diffraction revealed the important role of metastable assemblages in the subduction of slabs and consequent material exchanges between upper and lower mantles. Further development of the experimental and analytical technique is expected in the research fields of deformation mechanics and element diffusion for Earth's materials.