The physical properties of the materials used are also important in the thermal conduction, besides many other factors. In this study, nettle fiber/polyester composites were formed using stinging nettle grown in the Black Sea region. The stinging nettle fibers used in the formation of these composites were divided into three parts as bottom, middle, and top. The physical properties (diameter, density, crystallinity) of the fibers obtained from different parts of the plant and how the increased fiber concentration affected the thermal conductivity coefficients of the composite materials formed were studied. As a result, it was observed that the thermal conductivity coefficients of the composites increased with the increase of the crystallinity ratio of the fiber. Moreover, the increased fiber concentration significantly increased the thermal conductivity coefficient of the composite materials produced.