Thermal fatigue cracks have been found at tee pipes where high and low temperature fluids flow in. In this study, mock-up tests and analyses were conducted to develop a monitoring procedure for the wall temperature fluctuation by temperature measurement of the outer surface at the mixing tee pipe. In the tests, the temperatures on the inner and outer surfaces at the mixing tee pipe were measured with thermocouples and thermography, respectively. Both the distributions of the time-averaged temperature and the temperature fluctuation range on the outer surface measured by thermography were similar to those on the inner surface measured with the thermocouples. The temperature fluctuation range on the inner surface was estimated using the developed inverse analysis and the temperature on the outer surface measured by thermography. It was also showed that the inverse analysis which considers the thermography resolution has to be used in order to estimate the temperature fluctuation range well.