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• Effective thermal diffusivity and conductivity of pebble bed are measured up to 1200 °C under vacuum and helium conditions. • Two vacuum tests and two helium tests are conducted to verify experimental repeatability and validity. • An improved method is used to retrieve results by alleviating wall effect. • Effective thermal conductivity is comparable with previous results of SANA and HTTU. Effective thermal diffusivity and conductivity are two crucial parameters to determine inherent safety in porous pebble bed of High Temperature Gas-cooled Reactor. A full-radius-scale heat test facility has been developed to measure these thermal properties at high temperature. Two vacuum tests and two atmospheric-pressure helium tests up to 1200 °C have been conducted to determine the temperature-dependent parameters by the inverse method. Repeatable results are validated by both tests with different heating processes. Furthermore, the primary experimental errors from installation positions of thermocouples as well as random packing effect are reduced by retrieving positions in the inverse method. Sensitivity and uncertainty analyses are carried out in respective tests to give the reasonable descriptions of the results of effective thermal diffusivity and conductivity in these tests. [ABSTRACT FROM AUTHOR]