The ordered intermetallic PtM alloys with L10 structure are considered as the most promising oxygen reduction reaction (ORR) catalysts owing to the strong chemical bond between Pt and M atoms effectively suppresses the leaching of M atoms. However, the preparation of ultrafine L10-PtM alloy nanoparticles with high ordering degree is still a challenge because the nanoparticles tend to agglomerate during high-temperature annealing. Herein, we propose a coating layer-free strategy to controllably synthesize sub-4 nm highly ordered intermetallic L10-PtCo catalyst. The strong interaction between N-doped carbon (NC) and PtCo(OH)x effectually inhibits the agglomeration of nanoparticles during thermal treatment, while the O vacancies generated by the thermal decomposition of PtCo(OH)x are beneficial to prepare the highly ordered intermetallic L10-PtCo alloy. The as-obtained L10-PtCo/NC alloy catalyst shows excellent ORR activity in both half-cell and single cell tests, which is much better than that of disordered A1-PtCo/NC and commercial Pt/C catalysts. This work provides a facile and efficient method for fabricating the ordered intermetallic L10-PtM alloy catalysts with high ordering degree.