The heat transfer coefficient of the interface between the fuel and the cladding of fast reactor fuel rods with different burnups was derived through the analysis of the structure of the UO2-PUO2 fuel A decrease of the heat transfer with increasing burn-up was found, resulting in rising fuel surface temperatures. The predictions of a theoretical heat transfer model agreed well with the experimental result. The deterioration of the heat transfer could be explained by fission gas release into the residual gap between fuel and cladding. Heat transfer through contact spots played a negligible role due to low contact pressure and very early formation of an oxide layer on the cladding.