Corncob powder filled high-density polyethylene (HDPE) composites were prepared by extrusion. The microstructure, water absorption, mechanical properties, and crystallinity of composite at different corncob powder content were investigated. Results demonstrated that when the corncob powder levels were moderate and uniformly dispersed within the HDPE matrix, the powder acted as a reinforcing agent. As the corncob content increased, the water absorption of the resulting composite gradually increased, which adversely affected the composite’s water resistance. Flexural strengths and moduli initially increased with increasing corncob powder levels, and then consequently decreased at higher powder levels; maximum values for flexural properties were achieved at 40% corncob powder content. The composite’s impact strength and toughness weakened with corncob powder addition. The X-ray diffraction and differential scanning calorimetry analyses indicated that when the corncob content increased, the peak crystallization and melting temperatures of the matrix increased and decreased, respectively. Meanwhile, the presence of the corncob restricted the movement and arrangement of the HDPE polymer chains, which affected HDPE crystal growth and causing a decrease in crystallinity.