A device and method for measuring the P-V relation of certain substances under hydrostatic pressure up to 30,000 kg/cm2 are described in detail.The pressure in the vessel was calibrated by the pressure transition phenomena of KBr, KC1 and Bi at 20℃. The result shows that the pressure in the vessel varies linearly with the applied force. Corrections were made for the deformation of the piston and the vessel by comparing the apparent volume contraction of a S. P. iron specimen measured in this vessel, with those reported by P. W. Bridgman. Some P-V relation of paraffin, pyrophillite, polystyrene, plexy-glass, Al, KBr, KC1, and Bi is also given.