Mixture of Ni(OH)2–Mg(OH)2 as precursor of the catalyst used to synthesize carbon nanofibers (CNFs) was treated by mechanochemical (MC) and hand-grinding process. It was found that catalyst particle size decreased sharply under grinding for 60min, and crystallinity of catalyst particle decreased under grinding. CNFs were synthesized by pyrolysising C2H2 using catalyst treated with MC process (labeled as ground catalyst) and hand ground (labeled as unground catalyst). CNFs with thin and uniform diameter were synthesized using catalyst treated with the MC process, which could be controlled by tuning the grinding time. In addition to tailor size and uniformity of as-grown CNFs, CNFs with bundle and twisted conformation were synthesized using catalyst treated with the MC process. After purification of CNFs and loading with Pt, they were used in fuel cell as the cathode catalyst support. The performance with CNFs synthesized using catalyst treated with the MC process was found to be better than that with CNFs synthesized using unground catalyst.