To examine the impact of nuclear transmutation in K-type and N-type thermocouples on temperature measurements in a fusion reactor, thermocouples with altered compositions were fabricated, and their responses were obtained at up to 800°C. The compositions of the thermocouples were altered according to transmutation calculations simulating the 3.5-, 4.6-, and 7-year use at the first wall and 40-year use at the front surface of the radiation shield. Comparison of the responses with commercial thermocouples at 800°C showed that the K-type and N-type thermocouples with altered composition simulating the 7-year use at the first wall indicate 20% to 25% lower temperatures. In this condition, the weight ratio of additive powders for simulation of transmuted elements was ~3%. The differences of responses between the commercial thermocouples and thermocouples simulating transmutation are dependent on the weight ratio of the additive powders. The present data could be used for estimation of response degradation of thermocouples used for long-term operation in a fusion reactor.