Raising ducks in paddy fields is a traditional agricultural model in China. Field experiments were conducted in rice-duck mutualist ecosystems in the double rice cropping region of Hunan Province during May to October of 2010. A conventional rice field was used as the control. The characteristics of nutrient return and uptake by rice in the rice-duck mutualism ecosystem were analyzed. The main objectives of the study were to provide theoretical and practical bases for further development and utilization of this classical agricultural technique. Results showed that feeding duck in paddy fields increased, respectively, carbon (C), nitrogen (N) and phosphorus (P) by 229.87 kg·ha^-1, 18.22 kg·ha^-1 and 17.75 kg·ha^-1 of duck feces during double rice cropping season. During double rice cropping season, the return of C, N and P to soil in the rice-duck mutualism ecosystem was in the order of C > N > P. The measured values of C, N and P were 1,491.21 kg· ha^-1, 66.02 kg· ha^-1 soils under the rice-duck mutualism ecosystem increased by 20.43% (P > 0.05), 55.81% (P < 0.05) and 379% (P < 0.05), respectively. While C and N returns were largely from rice root system, P return was mostly from duck feces. In contrast with conventional rice system, rice-duck symbiosis system increased soil total N by 5.73%, significantly increased soil total P by 6.25%. The rice-duck symbiosis increased root and straw total N and P and seed total P during double cropping rice season. It also increased straw N and P uptake, root P uptake and decreased yield and seed N and P uptake during double cropping rice season. Rice P uptake and content were better than N under rice-duck mutualism ecosystem.