Thermal distortion of mirrors caused by high power CW laser radiation and pressure distortion by water-cooled back pressure have been measured using a Twyman-Green interferometer. Analysis of measurement results show that it is necessary to reduce the radiation power density and increase the diameter-thickness ratio of mirrors in order to minimize the thermal distortion by laser and pressure distortion. SiC is a good substrate material for laser mirrors with higher thermal conductivity, higher Young’s modules and lower coefficient of thermal expansion.