Conventionally, lasers are designed to operate at the middle of thermally stable zones, where the fundamental mode size is insensitive to thermal perturbation, but is inversely proportional to width of stability zone, which will give rise to inconvenience or even difficulty in practice when large mode size is required. We propose a new simple approach, namely thermally-near-unstable resonator. The laser is designed to operate at the border of stability zone instead, where it has large fundamental mode size at gain media. With increase of pump power, mode size would grow up automatically to a value suitable for monomode operation. Stability of cavity on whole pump range can also be easily guaranteed. And there is a point where the laser power is insensitive to driving perturbations. However, the laser beam quality is sensitive to driving and thermal perturbations for the mode size depends severely on thermal focusing. Large-scale improvement in beam quality is demonstrated experimentally.© (2003) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.