Thermal stresses of thick-walled components in power plants are the limiting factors for rapid start-up, shut-down or load changes. In current industrial practice, thermal stresses are calculated from differential temperature measurements. In this paper a method is described to calculate thermal stresses from readily available plant measurements. Mathematical models of the critical thickwalled components were used for unit 8 at the GKM power plant in Germany in order to reduce the instrumentation costs and improve the quality of the thermal stress calulation. Operational experience has shown that the estimated inner and mean temperatures are considerably more accurate than the previously used measured values.