The influence of alkaline aqueous solution treatment of groundnut shell–high density polyethylene composites consist of magnesium hydroxide as flame-retardant additives was investigated. FTT daul cone calorimetry and Perkin-Elmer Diamond differential scanning calorimetry (DSC) were used to characterize flammability and thermal properties of the fabricated composites. Quanta 200ESEM was used to investigate the effect of alkaline aqueous solution treatment on the microstructure of the composites. The result obtained revealed that the alkaline treatment of the shell enhanced filler–matrix compatibility. The cone calorimetry result revealed that Mg(OH)₂ has good fire retardancy characteristics on the fabricated composites as properties such as heat release rate, effective heat of combustion, mass loss rate, total smoke produced, and specific extinction area were enhanced. DSC results suggested that the additives had little impact on the melting temperature of the composite.