Abstract: Thermal effect is the main reason limiting output power and beam quality of vertical-external-cavity surface-emitting lasers (VECSELs). To optimize the quantum design in active regions of gain chip, decrease thermal effect and upgrade slope efficiencies and output laser power, thermal characteristics of a VECSEL at 980nm wavelength were investigated experimentally by means of photoluminescence. The edge-emitting and surface-emitting spectra at different heatsink temperatures were obtained. The results indicate that the redshift rate of the edge-emitting spectra, which revels the intrinsic properties of quantum wells in active region, is 0.5nm/K with increasing temperature, while the redshift rate of the surface-emitting spectra, which are modulated by multiple-layer structure of the gain chip, is only 0.1nm/K with increasing temperature. The experiments also show that the surface-emitting spectra are split into several modes because of the restriction from the micro-cavities in gain chips of VECSELs. In the design of VECSEL gain chip, according to the above thermal characteristics of VECSELs, performance of a laser could be improved significantly by prebiasing emission wavelengths of quantum wells and cavity lengths of micro-cavities.