The microstructure and mechanical properties of pure ultra‐high molecular weight polyethylene (UHMWPE) and UHMWPE/CNF (carbon nanofibers) composites were investigated under different aging time based on differential scanning calorimetry, Fourier transform infrared spectroscopy, scanning electron microscopy, uniaxial tensile, and creep tests. The results showed that the aging mechanism of both pure UHMWPE and UHMWPE/CNF composites was thermal oxidation of long carbon chains at low temperatures, and CNF did not participate in the aging reaction. However, the addition of CNF increases the oxidation reaction area, which makes the aging of UHMWPE/CNF composites faster and more complete. Compared with pure UHMWPE, the Young's modulus of the composites with CNF addition was significantly higher, but the tensile strength and elongation were lower. The total creep strain of the composites decreased with increasing aging time. In addition, an aging‐degree‐dependent viscoelastic‐plastic creep model is proposed on the basis of the Kelvin viscoelastic model and Zapas‐Crissman viscoplastic model, which is in good agreement with the experimental data.