<p>This research explores the advancement of hybrid kenaf/epoxy composites, incorporating crude oil waste (pet coke) filler at different loadings (1, 3, and 5 wt%), in response to the increasing demand for sustainable composite materials. Both untreated and sodium-bicarbonate-treated kenaf fibers were utilized to assess the synergistic effects of surface modification and filler incorporation on mechanical, thermal, and water absorption characteristics. The integration of rigid pet coke particles markedly improved the performance of the composite. The treated kenaf epoxy composite with 5&#xa0;wt% filler (TKE5) demonstrated superior tensile strength (35.87&#xa0;MPa), flexural strength (72.17&#xa0;MPa), and impact strength (27.17&#xa0;J/m), reflecting enhancements of 13.9%, 42%, and 44%, respectively, compared to untreated kenaf fiber composites. The water absorption of TKE5 was maintained at less than 1% after 216&#xa0;h, accompanied by a diffusion coefficient of 3.28 × 10⁻¹²&#xa0;m²&#xa0;s⁻⁻¹, which suggests exceptional moisture resistance. Thermogravimetric analysis (TGA) demonstrated enhanced thermal stability, evidenced by a residual char yield surpassing 30&#xa0;wt% at 600&#xa0;°C. Furthermore, all composites met the UL-94 HB flammability standard, exhibiting burning rates below 25.2&#xa0;mm/min. The application of scanning electron microscopy (SEM) demonstrated enhanced fiber–matrix interfacial bonding and a more uniform distribution of fillers within the treated composites. The findings indicate that the application of sodium-bicarbonate treatment alongside the use of pet coke filler presents an economical and environmentally sustainable approach to the fabrication of mechanically resilient, thermally stable, and flame-retardant kenaf/epoxy hybrid composites.</p> Graphical Abstract <p></p>