Abstract: In this work, calcium hypophosphite (CaHP) was introduced into polylactide to prepare flame retardant polylactide composites (FR-PLA). Thermal gravimetric analysis (TGA) was used to investigate the thermal degradation properties of CaHP and FR-PLA composites. It was found that CaHP presented a different thermal degradation process in air atmosphere compared with that in nitrogen. The flame retardancy properties of the composites were evaluated by limiting oxygen index (LOI), underwriter laboratories 94 testing (UL-94), microscale combustion calorimetry (MCC) and cone calorimetry. LOI and UL-94 testing showed that the incorporation of CaHP can significantly improve the flame retardancy of FR-PLA composites. MCC and cone calorimeter tests showed that the values of heat release rate (pHRR) and total heat release (THR) of FR-PLA composites were significantly decreased compared with those of pure PLA. The thermal degradation processes of PLA and FR-PLA were researched by real time Fourier transform infrared spectra (RT-FTIR). The crystallization behavior of FR-PLA composites was investigated by differential scanning calorimetry (DSC). Scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR) revealed that the addition of CaHP promoted the formation of condensed char residue which contained aromatic structure and phosphorous-rich inorganic structure. The condensed char could inhibit the inner material from being attacked by the heat flux and flame during combustion, resulting in improved flame retardance of FR-PLA composites.