A new type of waveguide CO2laser with a previously proposed waveguide structure and a uniform transverse but horizontal (parallel to the waveguide surface) excitation is reported. The waveguide consists of a pair of slightly curved parallel plates which have a low‐loss transmission and a self‐focusing property; flat mirrors are set directly at both ends of the waveguide. The laser provides 30 W peak power at the repetition frequency of 3 kHz at pressures as high as 250 Torr. The output beam is 70% horizontally polarized, indicating laser oscillation mainly in the TE1waveguide mode within the resonator.