The aim of this study was to investigate the effect of nanoboron nitride (BN) as nano-fire retardant on some properties of polylactic acid (PLA) and polyhydroxybutyrate (PHB) biopolymer nanocomposites reinforced with cellulose nanofibrils (CNFs) and nanoclays (NC). BN particles, as nano-fire retardant, were added to enhance the thermal stability of the obtained biopolymer nanocomposites. PLA and PUB nanocomposites were prepared with a twin screw extruder and characterized with mechanical tests, TGA, and DSC. Densities of the PI A and PUB nanocomposites were found to decrease with the addition of NBN loading. The mechanical properties of biopolymer nanocomposites decreased, except for tensile modulus. A ccording to TGA results. NBN loadings generally improved the thermal stability of the PLA and PUB nanocomposites. The degradation temperature for weight loss at 10 %, 50 % and 85 % (T-10% , T-50% and T-55%) increased with NBN loadings: the value of DTCmax was determined to improve with the loading of BN. DSC results showed that melt temperature (T-m ) and crystallization temperature (T-c) generally increased with BN loadings, whereas crystallinity (Xc) decreased with BN loadings in PLA nanocomposites.