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Due to the unique two-dimensional structure and features of graphitic carbon nitride (g-C₃N₄), such as high thermal stability and superior catalytic property, it is considered to be a promising flame retardant nano-additive for polymers. Here, we reported a facile strategy to prepare cobalt/phosphorus co-doped graphitic carbon nitride (Co/P-C₃N₄) by a simple and scalable thermal decomposition method. The structure of Co/P-C₃N₄ was confirmed by scanning electron microscopy (SEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The carbon atoms in g-C₃N₄ were most likely substituted by phosphorous atoms. The thermal stability of polylactide (PLA) composites was increased continuously with increasing the content of Co/P-C₃N₄. In contrast to the g-C₃N₄, the Polylactide (PLA) composites containing Co/P-C₃N₄ exhibited better flame retardant efficiency and smoke suppression. With the addition of 10 wt % Co/P-C₃N₄, the peak heat release rate (PHRR), carbon dioxide (CO₂) production (PCO2P) and carbon oxide (CO) production (PCOP) values of PLA composites decreased by 22.4%, 16.2%, and 38.5%, respectively, compared to those of pure PLA, although the tensile strength of PLA composites had a slightly decrease. The char residues of Co/P-C₃N₄ composites had a more compact and continuous structure with few cracks. These improvements are ascribed to the physical barrier effect, as well as catalytic effects of Co/P-C₃N₄, which inhibit the rapid release of combustible gaseous products and suppression of toxic gases, i.e., CO.