Rf excited two-channels waveguide CO₂ laser with the construction of unequally long electrode is present. The offset frequency of laser fr om the two channels can be changed by adjusting the RF input power, the slope of the tuning is about 0.1 MHz/W. The offset frequency stability of short term ca n be up to the order of 10-10 without any additional measure being taken to stabilize the frequency. In addition, a theoretical analysis is made for the o ffset frequency being tuned by RF input power.