In this study, a new type of carbon support materials for proton exchange membrane fuel cells (PEMFCs) was evaluated. Carbon nanofibers (CNFs) grown on activated carbon fibers (ACFs) were prepared through catalytic growth to yield CNF/ACF materials. CNFs were synthesized by using CH4 with Ni catalysts dispersed on ACFs. The as-prepared samples were characterized with transmission electron microscopy (TEM) and scanning electron microscopy (SEM). SEM images revealed a three-dimensional CNF network grown on the ACF surface. High-temperature treatment effectively reinforced the CNF/ACF structure and led to increased electrochemical performance and durability of Pt catalysts in PEMFC operations. The accelerated degradation test (ADT) was used for stability evaluations of Pt catalysts. The ECSA loss from heat-treated Pt/CNF/ACF at 900°C was calculated to be 39.1%, whereas the ECSA loss from the non-heat-treated Pt/CNF/ACF rose to 56.3%. The results suggest that the higher corrosion resistance of the carbon support could come from the higher degree of graphitization through high-temperature heat treatment of CNF/ACF. This unique structure of the CNF grown on ACF can supply effective anchor sites for the stabilization of Pt particles. As a result, the heat-treated CNF/ACF is expected to be a promising carbon support to improve the cell performances of PEMFCs.