We analyzed the effects of rice straw biochar (RSBC) and swine manure biochar (SMBC) on NO emission from paddy soil. The biochars were added to soil at the rates of 1% and 5% (w/w), and NO emission, soil properties and soil enzyme activities were determined at the elongation, heading and maturation stages of rice growth. The NO flux started within 2 h of adding the biochar, and decreased significantly thereafter during the three growth stages. The cumulative NO emission was suppressed by 45.14-73.96% following biochar application, and 5% SMBC resulted in the lowest cumulative emission. In addition, biochar application significantly increased soil pH, soil organic carbon (SOC), NO levels and urease activity, and decreased soil NH and nitrate reductase activity. Regression analysis indicated that cumulative NO emission was correlated positively to NH, and negatively to soil pH, SOC and NO. SEM further revealed that biochar application weakened the denitrification process, and the NH level had the most significant impact on NO emission. Taken together, RSBC and SMBC regulated the nitrogen cycle in paddy soil and mitigated NO emission by increasing soil pH, decreasing nitrate reductase activity and NH content.