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As the fundamental components of polymer electrolyte membrane fuel cells, the thermal properties of gas diffusion layers (GDLs) play a significant role in influencing their overall heat conduction and management. We have experimentally characterized the temperature dependence of the in-plane thermal conductivity of a commercial carbon paper GDL, including the progressively in-plane thermal degradation of GDLs through a series of repetitive freeze-thaw thermal aging cycles. Even though the thermal properties of carbon fibers are crucial for the thermal performance determination of GDLs, the temperature-dependent in-plane thermal conductivity of pristine GDLs depends mainly on the thermal characteristics of the binder that connects carbon fibers and the polytetrafluoroethylene (PTFE) coated on the microporous layer. The microstructural failure of aged GDLs (breakage of carbon fibers, delamination, adhesive failure of binder, and PTFE cracks) caused by the freeze-thaw thermal aging treatment dominantly affects the in-plane effective thermal degradation at different thermal aging phases. Overall, the breakage of carbon fibers positively contributes, and delamination, adhesive failure of binder, and PTFE cacks negatively contribute to the in-plane effective thermal conductivity of GDLs. In particular, the in-plane thermal conductivity of aged GDLs through 300 cycles reaches a peak.