Abstract The aim of this study was to investigate the thermal conductivity of natural fiber reinforced polymer composites (NFRP) as potential structural materials. As a natural fiber, Bermuda grass seeds, conifer cones and pinecones are selected. The matrix comprised Vinyl ester resin. The mechanical properties (tensile strength and Young’s modulus) and fractography analysis were investigated in our previous study (Dikici B. M. S.-H., 2019). In the current study, the thermal conductivity was probed using transient plane source technique implemented in the TPS 2500S Thermal Constants Analyzer. The addition of 9% Bermuda fibers yielded a decrease of approximately 19.3% in thermal conductivity compared to that of the neat epoxy. With the addition of 9% nano cellulose fibers, a decrease of approximately 40.7% in thermal conductivity was observed in the nanocellulose/vinyl ester resin composite compared to the neat vinyl ester samples.