The dry density and thermal conductivity of foamed concrete, of which dry density is less than 300kg/m3 were tested. The air void structures of the foamed concretes were investigated by X CT. The tested results were analyzed by using theoretical models for thermal conductivity of two phase materials. The new equation was proposed and verified and it was applied to foamed concrete of which dry density is less than 300kg/m3. The results indicate that the predicted values agreed well with experimental ones.