The effect of the structural eccentricity on the output power of a coaxil-c
avity gyrotron oscillator is investigated by making use of a gyrokinetic de
scription. It is found that the output power will be substantially weaken b
y the structural eccentricity of the inner-rod axis to the outer-cavity axi
s. The influences of the operating magnetic field and the electron-beam ene
rgy on the output power are analysed with the consideration of the structur
al eccentricity.