Based on the long-term datasets of soil organic matter content and the observation data of rice paddies' methane (CH4) emission collected from the agricultural experiment stations across the country, the rice paddies in China were divided into single cropping and double cropping regions. The soil carbon sequestration potential of straw return in three types of rice paddies in the two regions, i. e., single cropping rice paddies, upland/paddy alternated rice paddies, and double cropping rice paddies, was evaluated, based on the datasets of soil organic matter content; and the total CH4 emission from rice paddies without straw return was estimated, with reference to the experimental data of paddies' CH4 emission and by the method of mean emission coefficient. The total CH4 emission from our paddies after straw return and the global warming potential of the increased CH4 emission were also estimated by using the related methods and parameters given by IPCC. It was estimated that the full popularization of straw return to China's rice paddies would sequester 10.48 Tg x a(-1) of C, and the contribution to the global warming mitigation was 38.43 Tg CO2-eqv x a(-1). In the meanwhile, the CH4 emission from our rice paddies would be increased from 5.796 Tg x a(-1) to 9.114 Tg x a(-1), and the increased 3.318 Tg x a(-1) of CH4 emission would lead to a global warming potential of 82.95 Tg CO2 -eqv x a(-1), which was 2.158 times of the mitigation from carbon sequestration in rice paddies. Therefore, the increased CH4 emission due to straw return should be regarded as an important greenhouse gas leakage, since it could greatly offset the mitigation benefits of soil carbon sequestration in China's rice paddies.