Erosion of a hafnium cathode in Plasma Arc Cutting torch using oxygen as plasma gas is considered. It is shown that approximately 0.001 fraction of the evaporated particles participate in a net erosion, the rest of the evaporated particles return back to the cathode after spending some time in a near-cathode plasma. Along with erosion rate, the suggested equations allow one to the calculate current density at the cathode, the cathode temperature inside the arc attachment and the electron temperature at the cathode-plasma boundary. Comparison of the obtained values with the available information on these parameters shows a reasonable agreement.