Abstract: The canopy CO2 flux over rice-rape succession system in Jianghan plain was measured during 2010 and 2011 based on eddy covariance system, the daily and seasonal variation of CO2 source and sink over cropland ecosystem was analyzed. The results showed that daily variation of CO2 flux was obvious. The daily CO2 flux variation of NEE (Net Ecosystem CO2 Exchange) was obtained by using interpolated method. The average NEE in the stages of mid season rice transplanting, tillering, jointing-heading and milky were 1.63, -11.86, -20.61, -4.65g〖KG-*6〗·〖KG-*6〗m-2〖KG-*6〗·〖KG-*6〗d-1 respectively, and the average NEE in the stages of idle, rape seedling-bud, bolting, flowering and pod bearing were 2.18, 0.43, -5.00, -11.70, -13.91g〖KG-*6〗·〖KG-*6〗 m-2〖KG-*6〗·〖KG-*6〗d-1 respectively. Rice-rape succession ecosystem absorbed 19.26tC〖KG-*6〗·〖KG-*6〗ha-1 CO2 from atmosphere during the whole year.