An experimental investigation has been made on the turbulent thermal boundary layer in the entrance region of a smooth pipe with a uniform wall temperature. The temperature profiles in the entry region are well described with the logarithmic "law of the wall". The relation of universal temperature to universal velocity becomes linear in the turbulent part of the wall region. The empirical formula of the eddy diffusivity for heat has been derived from the measurements of the eddy diffusivity and the turbulent Prandtl number in the wall region. Inseparably associated with the large-scale variation of the turbulent boundary layer thickness with time, the thermal boundary layer in the entry region fluctuates with irregular frequencies less than 2 Hz, and this has a direct effect upon the mean temperature profile.