The performance characteristics of a 100MHz-RF-excited slab CO₂ laser are shown in terms of resonator setup conditions and discharge gap spacings. On varying the gap spacings from 2.25 down to 1.5mm, the optimum gas pressure shifted in such a way that the product of pressure and gap spacing remains constant in our experimental conditions. The optimum gas pressure for each gap spacing increased with RF input power. A laser output power of 210W was obtained with a corresponding conversion efficiency of 10.5 % and with a near-diffraction limited beam quality showing beam divergence angles of 5.1 and 1.7 mrad for stable and unstable directions, respectively. The experimental results obtained are discussed in details from the viewpoints of the E/P (electric field/pressure) value, sheath width in discharge plasma, and average gas temperature of the RF discharge region.