As one of the main countries in rice production, large quantities of rice straw produced in southern China each year are often directly returned to paddy field. This practice usually results in a significant increase in CH₄ emission. Biochar, the highly aromatic substance remaining after pyrolysis of plant biomass under complete or partial exclusion of oxygen, might play an important role in carbon mitigation as a soil amendment. Conversion of rice straw to biochar and adding it to soil may play an important role in carbon sequestration and reducing pollutants emission from paddy soil in China. In this study, rice straw biochars carbonized at different pyrolysis temperature and holding time were characterized and used for assessing their effects of biochar addition on GHGs emission, nitrogen retention and leaching characteristic in flooded paddy soil. Results demonstrated that peak temperature has an important effect on the properties of rice straw biochars compared to holding time. 500 °C is high enough for rice straw to carbonize for biochar which is highly aromatic and aromatic function amount to 98.6%. Adding biochar could dramatically reduce CH4 emission from the paddy soil as compared with the direct returning of rice straw. Biochar amendment could also retard the vertical movement of NH4+-N in soil layer Our results indicated that returning biochar pyrolysized from rice straw into soil might be a potential technology in increasing carbon sequestration, mitigating CH4 emission and reducing nutrient run-off in paddy soil.