Abstract: The invention relates to a method of measuring the RF output power (Ps) of a microwave tube amplifier, the tube having an electron gun delivering an electron beam, an RF circuit for interaction between an RF signal and the electron beam, the RF circuit having an amplified RF signal output, and a collector having at least two electrodes for collecting the electron bream. The RF output power as amplified RF signal output is determined from the measurement of the current Ic1 coming from the first electrode closest to the gun, a calculation of the RF output power (Ps) being carried out through a predetermined relationship between said current and the output power of the amplifier. The invention also relates to a device that includes measurement means for implementing the measurement method.