The optical cavities of general curved-mirrors with a negative lenslike medium are discussed, and used to analyze the thermal lensing effects in gas lasers. The mode analysis is made for a CW CO2 laser of a plane-concave cavity. It is shown that the thermal lensing effects have considerable influence on the output and resonance frequencies, particularly for the fundamental mode lasers. Finally, possible thermal lensing effects in ion gas lasers and other lasers are briefly discussed.