Yttrium aluminum garnet (YAG, Y3Al5O12) doped with rare earth ions is one of the most common active media in solid state laser materials. In particular in high-power lasers, thermal management is a crucial issue, requiring information on the thermal properties. The doping of YAG with rare earth ions occurs by the substitution for Y3+ ions, leading thus to a single phase system, for which the thermal conductivity cannot be estimated by the means of rigorous bounds. In this work the thermal diffusivity and conductivity of polycrystalline YAG ceramics doped with rare earth ions were measured. Transparent fully dense ceramic samples of YAG doped with various concentrations of Yb and Er were prepared by solid state reactive sintering of oxide powders. The sintering was performed under high vacuum with the addition of TEOS as sintering additive. Thermal diffusivity of sintered ceramics was measured by laser flash method at temperatures from room temperature to 900 °C. The effect of radiation is discussed, in particular at higher temperatures. The obtained results show clear trends with dopant concentration and temperature. Materials with layered doping concentration distribution were also tested and the results are confronted with respective model predictions. The measurement results are confronted with literature data and with empirical relations.