This study explores the thermal behavior of HDPE composites reinforced with micro- and nano-Hydroxyapatite (HA) through Thermo Gravimetric Analysis (TGA) and Differential Scanning Calorimetry (DSC). TGA was employed to assess thermal stability, with tests conducted from 25 to 550°C. The analysis revealed that HDPE/micro-HA composites begin to degrade at around 300°C, with polymer decomposition complete by 450°C. Micro-HA demonstrated higher thermal stability, increasing degradation temperatures more than nano-HA due to its larger size and surface absorption properties. DSC analysis measured the effects of filler content on the thermal properties, including crystallization temperature (Tc) and melting temperature (Tm). The results showed no significant change in Tm between pure HDPE and HDPE-HA composites, indicating that HA had little impact on HDPE crystallization. However, as the filler content increased, the melting temperature of the composites slightly increased. The crystallinity of HDPE-HA composites improved up to 5 wt% micro-HA and 1 wt% nano-HA, after which crystallinity declined. The findings suggest that both micro- and nano-HA enhance the thermal properties of HDPE, but their effectiveness varies with filler concentration.