The thermal conductivities of many building materials are defined in the TS 825 standard. In this standard, the thermal conductivity of wall units such as brick and lightweight block was determined depending on the mass per unit volume and their configurations. Additionally, these values are used in the heat loss calculations of the buildings. The thermal conductivities of walls built with lightweight masonry block at 450-700 kg m(-3) are given between 0.28 and 0.36 W m(-1) K-1 in TS 825 standard. The insulation thicknesses of building's external wall are directly affected by these values in different climatic regions of Turkey. In this study, the effect of foam concrete as filling material on thermal conductivity of lightweight masonry block was investigated. For this purpose, thermal conductivities of non-filling block and foam concrete-filled block were determined. The thermal conductivities of the block matrix and foam concrete were measured with HFM device. Then, thermal resistance and thermal conductivity values of blocks were analyzed by using computer program. Finally, the design thermal conductivities of external walls were calculated. When the analysis results were interpreted, it was found that the thermal conductivities of foam concrete-filled blocks are 4 times lower than the non-filling blocks. As a result, it was found that the use of foamed concrete at a density of 300 kg m(-3) in order to improve the thermal insulation performance of hollow blocks was found to be extremely effective.