Information about root respiration is essential for evaluating soil carbon cycling and sequestration in forest plantation ecosystems. Using LI-COR-6400-09 system, root respiration was determined in poplar plantations at three different developmental stages in Xinjiang. Based on the field observation in 2007 and 2008, the seasonal dynamics of root respiration and its main controlling factors including tree age, soil conditions and fine root biomass were analyzed. The results showed that root respiration rate of the plantations (aged from 2, 7 to 12) displayed similar seasonal patterns, with the maximum occurred in early summer and the minimum in autumn, closely followed the seasonal variation of soil temperature in the top layer. Furthermore, root respiration rate peaked earlier than soil temperature and fine root biomass in the growing season, which could imply that root seasonal phenology and belowground carbon allocation also played a role in variation of root respiration. The average root respiration in the 2-year-old stands was 3.78 μmol CO2 m-2 s-1, then declined with increasing plantation age. The contribution of root respiration to soil respiration varied from 38.6% to 58.0%, with the maximum occurred in the 2-year-old stands. The variability in root respiration with tree ages mainly resulted from the differents in root production, turnover and metabolism at these three stages. In sum, soil temperature and fine root biomass at top depth jointly explained 76% of variation in root respiration. Also, our results hightlight the importance of developmental stage in evaluating root respiration over a full rotation.