The fabrication of biodegradable polymer nanocomposites with improved flame retardancy has been an urgent task in practical because of the huge benefits of biodegradable polymers. In this work, 10-(2,5-dihydroxyl phenyl )-9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide ( DOPO-HQ)-functionalized graphene oxide (GO) (FGO-HQ) was used as a novel and highly efficient flame retardant (FR) to improve the flame retardancy of polylactide (PLA) nanocomposites. Contributed by the bi-phase flame retardant action, including the physical barrier char in solid phase and the decreased flammable volatiles in gas phase, the resultant PLA/FGO-HQ nanocomposites presented excellent flame resistance at the loading of 6 wt% FR: UL-94 reached V-0 rating; peak heat release rate (PHRR) and total heat release (THR) decreased by 24.0% and 43.0%, respectively; smoke production rate (SPR) and total smoke release (TSR) decreased by 46% and 83%, respectively. For further confirming its flame-resistance mechanism, thermogravimetric analysis/infrared spectrometry (TG-IR) and Fourier transform infrared spectra (FTIR), scanning electron microscope (SEM), and Raman spectroscopy were employed. Results indicated that the incorporation of FGO-HQ can effectively reduce the evaporation of flammable gaseous product in gas phase through quenching free radicals. Meanwhile, graphitized carbons are formed in the residual char and PLA/FGO-HQ sample can achieve a good thermal stability in the combustion with phosphorus containing compounds and aromatic structure in the solid phase. Furthermore, the tensile strength of PLA nanocomposites presented good mechanical properties with the addition of FR as well. These results suggested that the incorporation of FGO-HQ FR not only improve the flame retardancy and thermal stability of biodegradable polymer nanocomposites but also without sacrificing their mechanical properties. (C) 2018 Elsevier Inc. All rights reserved.