This study aims to determine the mechanical and thermal properties of High Density Polyethylene (HDPE) thermoplastic nanocomposite with Oil Palm Boiler Ash (OPBA) filler made by coprecipitation method and synthesized with PEG 6000 surfactant with OPBA - PEG 6000 filler variations. The filler composition used was HDPE/OPBA (100/0, 98/2, 96/4, 94/6, 92/8, 90/10) %wt. Nanocomposites were prepared using a Rheomixer HAAKE Polylab OS System at 150°C and 60 rpm for 10 minutes. Mechanical properties of HDPE with increased OPBA filler content is beyond a certain threshold, the tensile strength of the HDPE composite may start to decrease. This decrease can be attributed to several factors. Firstly, as the filler content increases, the HDPE matrix may become less effective in transferring stress, resulting in reduced load-bearing capacity. This is confirmed from the SEM results that the filler agglomerates and cracks occur in the composite. The composite material may exhibit a lower Young's modulus compared to pure HDPE with low Young's modulus tend to have high elongation at break which indicatie a flexible and ductile composite. The melting point of peaks 1 and 2 on 0% filler and other fillers did not change significantly even at certain compositions the melting point decreased after adding filler