In order to measure fluid temperature fluctuation accurately in turbulent flows by a fine-wire temperature sensor (cold-wire), we usually need to compensate for the sensor response appropriately based on its dynamic response characteristics. In the present study, we use a very fine temperature sensor of 0.25 μm in diameter to measure a turbulent thermal boundary layer flow developing over a uniformly heated flat-plate. We derive the more exact response transfer function based on the sensor geometry. Thanks to its extremely high response speed, the outputs of the ultimately fine-wire sensor do not need the response compensation. It is demonstrated that the power spectra, r.m.s. values and instantaneous signal traces of the temperature fluctuations can be measured accurately by comparing the results with those obtained by the response compensated cold-wire sensor of 3.1 μm in diameter.