For a radio-frequency (RF) excited slab CO2 laser the far-field spot is 0 form, the near-field profile is near a line form. The laser beam is more divergent in a certain direction on a plane perpendicular to the transmission direction, and cannot be used in laser material processing. According to the laser power and laser beam dimension, the near TEM00 mode with 6 mm diameter of a circular laser beam is got in a 200 W laser as well as the low order mode with 10 mm diameter of a circular laser beam in a 500 W laser, using defferent combination of circular cylindrical mirrors, diaphragm and beam-expanding telescope. The laser can be used in laser material processing.