The thermal conductivity coefficient lambda, has been measured in the temperature (T K) range 302.55-391.95 K for the cis and trans isomers of decahydronaphthalene and their commercial Fluka mixture (32.15% cis/67.85% trans)w/w. The experimental Setaram C80 D calorimetric device, equipped with a suitable vessel set-up, using the steady-state coaxial cylinders method, was calibrated and then tested on pure liquid toluene. The repeatability of the measurements is better than 1% [1], and the agreement with the values given in the literature enabled us to apply this method on the conformational cis-trans isomers of decahydronaphthalene. Thermal diffusion coefficients a, were calculated for each isomer from the conductivity results and the difference in their thermal properties is explained in compliance with the theoretical approach to organic liquid conductivity. Finally, a comparison between our experimental results and the data correlated by the Robbins and Kingrea equation confirms that experimental determinations are essential for conformational isomer structures.