In this study, two different carbons (synthetic graphite particles and carbon fiber) were added to nylon 6,6, and the resulting composites were tested for thermal conductivity. The first goal of this work was to compare through-plane thermal conductivity results from the guarded-heat-flow method and the transient-plane-source method. The results showed that both test methods gave similar through-plane thermal conductivity results for composites containing 10-40 wt % synthetic graphite and for composites containing 5-40 wt % carbon fiber. The advantages of using the transient-plane-source method were that the in-plane thermal conductivity was also measured and the experimental time was shorter than that of the guarded-heat-flow method. The second goal of this work was to develop and use a detailed finite-element analysis to model heat transfer within a carbon-filled nylon 6,6 composite sample for the transient-plane-source method and compare these results to actual experimental results. The results showed that the finite-element model compared well with the actual experimental data. The finite-element model could be used in the future as a design tool to predict the dynamic thermal response of different composite materials for many applications.