The influence of grain boundaries on heat transfer through polycrystalline alumina has been investigated between 20degrees and 500degreesC. The thermal conductivities of small-grained porous ceramics and large-grained dense ceramics have been measured using the laser-flash technique. Two methods have been developed to assess the average thermal resistance of a grain boundary. The first method is based on the comparison of room-temperature thermal conductivities for dense ceramics that have various average grain sizes. This method yields a value of 0.9 x 10(-8) m(2.)K(.)W(-1). The second method, particularly suitable for porous ceramics, is based on the extrapolation of the inverse of the thermal conductivity versus temperature to give an intercept with the axis at T = 0 K. This value is attributed to the thermal resistance of grain boundaries. By taking into account the influence of the pore content using an effective medium theory, the average thermal resistance of a grain boundary has been evaluated to be 1.3 X 10(-8) m(2.)K(.)W(-1) in dense alumina and 2.2 x 10(-8) m(2.)K(.)W(-1) in alumina containing a pore volume fraction of 0.3.