Abstract: The temperature fluctuation of copper surface,which was in a the rmal equilibrium state under the irradia ted condition with a CW CO2 laser,was carefully measured.And the maximum range of the temperature fluctuation was no relation to laser irradiation time.So the temperature fluctuation of the copper su rface showed considerably the laser power stability of 5kW CW transverse flow CO2 laser.The result was useful to control the power stability of a CO2 laser.