Precise size control and general synthesis of ultrafine noble metal-based catalysts are very important in electrochemical energy conversion technologies. Herein, we develop the ultrafine MCu (M = Pt, Au, and Pd) alloy nanoparticles anchored on the porous N-doped carbon nanofibers (MCu/p-NCNFs) by the electroless deposition method. The as-prepared MCu alloy nanoparticles possess ultrafine particles sizes (<10 nm) and are uniformly anchored on p-NCNFs, which are derived from electrospun polyacrylonitrile (PAN) nanofibers. Owing to the desired composition and structure, the as-prepared PtCu/p-NCNFs with an ultrafine alloy size (approximate to 3.5 nm) exhibits significantly enhanced electrocatalytic activity and stability toward methanol electrooxidation as well as the outstanding CO antipoisoning ability. This work offers a new general strategy for the fabrication of ultrafine alloy nanoparticles on a porous nitrogen-doped carbon support as highly efficient electrocatalysts for energy-related applications.