The influence of thermally induced phase-mismatch in nonlinear crystals for frequency doubling caused by absorption of laser power is described. A numerical model is developed, which considers the spatial temperature distribution in the crystal and the corresponding wave-vector mismatch. For the temperature profile an approximate analytical expression is derived from the the heat-transfer equation in cylindrical symmetry. The conversion efficiency is calculated by solving the basic differential equations for frequency doubling with a spatial dependent wave-vector mismatch. Because the absorption coefficients are rather different at the fundamental and second harmonic wavelength, the heat-density in the crystal depends on the conversion efficiency and vice versa. Therefore an iteration method has to be used to calculate self-consistent solutions. Experimentally a Q- switched oscillator-amplifier-system with an average power of 200 W at 1064 nm is frequency doubled to an output of 103 W at 532 nm using a KTP crystal. The numerical calculations are in good agreement with the experimental results.© (1997) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.