Aim of the present study was to correlate thermal conductivities of highly porous autoclaved aerated concrete (apparent density < 100 kg/m3 ) with appendant micro-and nano-structural properties. AAC specimens with approximate apparent densities, but different pore size distributions were fabricated through utilizing specially selected, surface active additives when preparing the slurry of raw materials, aluminum and water. The main focus of this study is on the quantitative analysis of size and morphology of both pores and solid skeleton by 3D X-ray tomography (CT) and scanning electron microscopy (SEM). Moreover, specifi c surface areas and pore size distributions were determined by the methods of gas adsorption and mercury porosimetry. Apparent thermal conductivities depend on the number of pores, pore size distributions and thicknesses of the separating walls of the solid skeleton. The present study provides further inside in the mechanisms of heat transfer in light weight autoclaved aerated concrete.