Abstract Reaction atmosphere is one of the principle parameters affecting pyrolysis characteristics of biomass. In this study, instead of feeding reactive gases, methanol was co-fed in the pyrolysis of saw dust, aiming to understand the impacts of methanol on evolution of functionalities/structures of the major pyrolysis products. The results showed that at low pyrolysis temperature of 350 °C, methanol did not affect much on the charring reactions but promoted the formation of the heavier organics. At 500 or 650 °C, the co-feeding of methanol enhanced the yields of both char and the heavy components of bio-oil, resulting from the reaction of methanol or its derivatives with the organic components on surface of biochar via recombination/condensation reactions. In addition, methanol enhanced evolution of the organics with π-conjugated structures. The co-feeding of methanol also enhanced crystallinity and sizes of the crystal carbon in the biochar, but led to their lower thermal stability. The in situ Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) characterization of saw dust pyrolysis indicated that methanol promoted the aliphatic structures formation on surface of the biochar, leading to the low thermal stability.