The nanocomposites of high density polyethylene (HDPE)/bagasse flour (BF) with different contents of the organomodified montmorillonite (OMMT) were produced by melt blending process. The thermal stability and combustion behavior of nanocomposites were characterized by thermogravimetric analysis (TGA), differential scanning calorimetry, and cone calorimeter tests. The results of TGA data of the nanocomposites indicated that the OMMT greatly enhanced the thermal stability, and char residues of the HDPE/BF blends gradually increased with increasing the OMMT content. The activation energy was determined to describe the energy consumption of the initiation of the thermal degradation process. The composites produced with the 6 phc OMMT had the highest activation energy values among the evaluated composites (106 kJ/mol), whereas composites without nanoclay exhibited the lowest one. Furthermore, as the OMMT was incorporated into the nanocomposites, the melting temperature (T-m), crystallization temperature (T-c) melting enthalpy (a dagger H-m) and crystallinity (X-c) of HDPE/BF blends increased. The findings showed that the OMMT effectively boosted the flame retardancy of nanocomposites due to the formation of the carbonaceous silicate char shields delayed time to ignition and the combustion process was remarkably hindered.