Biochar could be involved in the biogeochemical redox reactions of trace elements. In this study, the electron transfer processes during the reduction of Cr(VI) by biochar produced at different temperatures were evaluated. Biochar derived from peanut shell at 400-800^oC could effectively reduce Cr(VI) to Cr(III), and the reduction capability showed the decrease as the pyrolysis temperature increased from 400^oC to 600^oC, and then increase from 600^oC to 800^oC. The electron donating moieties of biochar experienced a transition from the O-containing functional groups, especially –OH in the form of phenol or alcohol at lower pyrolysis temperature (<600^oC) to conjugated π-electron systems associated with aromatic structure at higher temperature (>600^oC). Biochar could mediate the reduction of Cr(VI) by small molecular weight organic acid in soil such as lactate, oxalate, etc., enhancing both extent and reaction rate of Cr(VI) reduction. The reaction rate of Cr(VI) by biochar and lactate together was up to 16.3 and 345 times higher than that by either biochar or lactate alone, respectively. The mediation capability of biochar increased with pyrolysis temperature from 400^oC to 800^oC, which was due to the increased electrical conductivity of biochar related to the growth of aromatic clusters. Our results indicated that oxygen-containing functional groups and aromatic structure of biochar, which were greatly affected by pyrolysis temperature, could facilitate the electron donating and mediating processes in the reduction of Cr(VI), and thus influencing the transformation and fate of Cr in the environment.