AbstractBio-chars were produced by co-pyrolysis of lignin with high-density polyethylene at 350 °C, 450 °C, and 550 °C. X-ray diffraction (XRD), Raman spectroscopy, automated surface area and pore size analysis, scanning electron microscopy (SEM), Fourier transform infrared (FT-IR) spectroscopy, X-ray photoelectron spectroscopy (XPS), and electron spin resonance (ESR) spectroscopy were performed on bio-char to reveal the effect of temperature on its physical structure and chemical properties. All of the bio-chars demonstrated a highly disordered, turbostratic structure and exhibited a wide pore distribution. Dramatic losses of carbonyl, hydroxyl, and C-H groups indicated the development of condensed aromatic structure in the bio-chars. Specifically, biochar produced at 450 °C showed the highest degree of aromaticity, which is the relative content of aromatic structure with small fused rings and free radical concentration. This structure has more potential application in composite production and as solid fuel for its combustion or gasification. Moreover, biochar produced at 550 °C had the greatest porosity development, favoring its use as a precursor for activated carbon production.