In this study, the thermal, mechanical, and morphological properties of high-density polyethylene composites reinforced with wal1nut shell and nano-CaCO3 were investigated. Composites with a fixed walnut shell content (20%) and varying nano-CaCO3 concentrations (1%, 3%, and 5%) were prepared by extrusion and injection molding. The results showed that increasing filler content raised crystallization temperatures, while melting temperatures (Tm) slightly decreased, and crystallinity was lower compared to pure HDPE. The lowest Tm (128.2°C) and crystallinity (27.3%) were found in the PECCWS4 and PECCWS5 composites. Thermogravimetric analysis (TGA) revealed that the initial degradation temperature (T5) increased from 452°C to 457.8°C, and the temperature for 50% weight loss (T50) shifted from 481.8°C to 486.5°C. Residual weight at 600°C increased from 1.1% for PECCWS1 to 9.5% for PECCWS5, indicating improved char formation. Scanning Electron Microscopy and Transmission Electron Microscopy showed improved filler distribution and enhanced filler-matrix interaction. The combination of walnut shell and nano-CaCO3 improved certain mechanical properties such as tensile strength and modulus; however, elongation at break decreased in tensile testing, indicating reduced ductility. These findings highlight the synergistic effect of walnut shell and nano-CaCO3 in enhancing HDPE composites' thermal and mechanical properties, showing potential for improved performance.