To investigate the effect of biochar on dehydrogenase activity and iron reduction in paddy soil, twoSkinds of paddy soil in different areas were anaerobically incubated under the addition of biochar with different partical sizes. Changes of dehydrogenase activity, Fe(II) concentration and pH were determined during incubation. Results showed that biochar promoted dehydrogenase activity and microbial Fe(III) reduction. Dehydrogenase activity and iron reduction potential in paddy soils increased with the decrease of the biochar practical sizes. Dehydrogenase activity was 3.13 and 2.60 μg/(mL.g.min), and Fe(II) concentration was 8.07 and 7.44 mg/gwithout adding biochar in two paddy soils, respectively. With adding biochar, dehydrogenase activity arrived at 4.35 and 4.18 μg/(mL.g.min), and Fe(II) concentration was 9.01 and 8.18 mg/g. CCA showed a significant correlation between initial pH of paddy soils and iron reduction potential, which indicated that the effect of biochar on iron reduction was different due to the disparity of soil property. Dehydrogenase activity and Fe(II) concentration showed a significant correlation, and both of them promoted each other during incubation.