Abstract: Raw biomass and its de-potassium sample washed by water were pyrolyzed in a fixed bed reactor under pure N₂ and CO-rich atmosphere at different temperatures. The obtained biomass chars were characterized by Fourier transform infrared spectroscopy (FT-IR), surface area analyzer, inductively coupled plasma emission spectroscopy and polarizing microscope. The yields of char, functional groups and other physicochemical structure were also studied and their evolution behaviors were examined. The results show that below 750 ℃, the BET surface areas and the amounts of alkyl and aliphatic functional groups in the chars derived from raw biomass are higher than those of de-potassium ones, but the char yields and the amounts of aromatic functional groups in the chars of raw biomass are lower than that of de-potassium samples. As to the effect of CO atmosphere, the yields of char and the amounts of all functional group in the chars derived from CO-rich atmosphere are less than that of N₂ atmosphere, But the surface areas is higher than that of N₂ atmosphere. Above 750 ℃, both potassium and CO can help to increase the char yields but decrease the amount of functional group and BET surface areas. In addition, the minerals are highly scattered and theirs amount on the surface of chars (raw biomass char, N₂ atmosphere) is less when the pyrolysis temperature is lower than 750 ℃; while above 750 ℃, those are higher and the clusters of fusion mineral components increases with increasing pyrolysis temperature. As for the graphitization degree of biomass chars, it increases with increasing pyrolysis temperature but poor graphitization was obtained when the pyrolysis temperature is lower than 750 ℃. Moreover, CO can help to increase while K decreases the graphitization degree above 750 ℃.