There are many studies about the thermal conductivity coefficient of lightweight concrete using lightweight aggregates such as perlite, pumice, vermiculite, diatomite earth and expanded clay aggregates. However, researches related to the thermal conductivity of lightweight aggregates is limited. The lightweight aggregates are the main raw materials of lightweight construction material. In addition, the usability of lightweight aggregates as core material has been investigated in recent years. Thermal conductivity of lightweight aggregates significantly affects the thermal insulation performance of such composites. The thermal conductivity coefficients of lightweight aggregates are related to many of the physical properties of aggregates such as specific gravity, porosity, particle size etc. In this study, the effects of loose unit weight, the volume of aggregate matrix, porosity and grain size on the thermal conductivity of lightweight aggregates was investigated. Limestone aggregate (normal aggregate) was also used for comparison in the study.