To study the seasonal variation of root respiration of Fraxinus mandushurica and its controls in broadleaf-Korean pine mixed forest of Changbai mountains, monthly root respiration rate was measured using in vitro root method from May to October 2007, and air temperature, soil temperature, soil moisture and root N content were determined simultaneously. The results showed that the root respiration rate of four age classes had an obvious seasonal variation, with the maximum occurring in July and the minimum in October. The root respiration rate ranged between 1.69 and 9.74 μmol/(g•s) during the growing season. The root respiration rate increased with increasing air temperature, soil temperature and root N content. More soil water in a certain range improved root respiration rate, but excessive water in soil, on the contrary, restrained root respiration rate. The response of the root respiration to temperature differed with age class, and the Q10 value ranged from 1.54 to 2.89, which increased with increasing forest age.