The aim of the paper is to verify the effectiveness of the new technique of measuring the superheated steam temperature in the transient state in a power plant. It based on the application of a new design thermometer with calculation method using the solution of the inverse heat conduction problem. Steam temperature was also measured with an existing conventional thermometer so that the advantages of the new measurement technique could be assessed. Temperature measurements using both thermometers were made at the boiler start-up, boiler shut-off and during boiler operation. The thermometers were located downstream of the second superheater attemperator. The results of measurements were compared. The fastest change in steam temperature occurred after the sudden injection of water into the attemperator header. In this case, the temperature measurement errors with the conventional thermometer are the highest. Dynamic errors of steam temperature measurement using conventional thermometers exceeded 80 K. The measurements made show a much greater accuracy of the measurement of the transient steam temperature using the proposed new technique.