It is known that when the incident power is large, the extinction ratio of electrooptic light modulators declines due to the thermally induced birefringence accompanying the absorption of light in the electrooptical crystals. The decline of the extinction ratio is analyzed. The important characteristics of the decline of the extinction ratio are the following. 1) It is independent of the beam radius or the surrounding temperature. 2) The decline can be reduced by simple methods such as thermal compensation. The results obtained by the analysis agree well with the experiment. From these results, it can be concluded that electrooptical light modulators constructed with materials like KDP, ADP, etc., can handle considerably high input power. This effect places an upper limit on the amount of power that can be handled by electrooptic modulators using naturally birefringent crystals.