The pressure loss in a high pressure vessel of a piston‐cylinder press has been studied at the bismuth (I‐II) and (II‐III) transitions. This was accomplished by measuring the abrupt diminution of volume of bismuth, which was surrounded by a solid pressure transmitting medium, at the pressure‐induced phase transitions. The study of pressure loss has been made by measuring three pressure losses which are defined as follows: (1) Pressure loss I is due to the interfacial friction between the cylinder and the piston. (2) Pressure loss II is due to the interfacial friction between the cylinder and the solid pressure transmitting medium which separates the piston and bismuth. (3) Pressure loss III originates within ``the sample‐containing cross section of the cylinder'' and is due to the internal friction of the medium. Experiment showed that the pressure loss III is the largest among the pressure losses. The difference between the true pressure on the bottom of a sample and the average pressure on the top of the piston due to pressure loss III has been formulated. This pressure loss study has been applied to the pressure calibration of a high pressure furnace containing a sample under investigation and also to the drawing of a diagram of pressure distribution in the furnace.