It is believed that interference effect of Ge-window is an important factor affecting CO2 laser output staibilty. The two parallel surfaces of a Ge-window constitute a Fabry-Parot interferometer, so for an incident laser beam, the transmissivity depends sensitively on thickness as a result of interference. And the thickness of Ge-window varies with laser intensity and ambient temperature. Therefore it is suggested that this may constitute a important cause of instability of the power output, frequency and transverse mode.