Viscosity is one of the most fundamental physical properties controlling migration processes of magmas and liquids in the Earth's interior. Here we introduce recent advances in viscosity measurement of liquids under high pressure and high temperature conditions. Capability of viscosity measurement using the falling sphere technique combined with large volume press predominantly depends on speed of X-ray imaging. Recent development of ultrafast X-ray imaging with more than 1,000 frames/second imaging rate opens a new way to investigate viscosity of low viscous melts such as carbonate melts and molten salts at in situ high pressure and high temperature conditions.