Root respiration supplies energy for root growth, maintenance and ion uptake. We studied the influence of phenology, temperature, root size and nitrogen fertility on root respiration rate of Fraxinus mandshurica Rupr. seedlings using a Li-6400-06 leaf chamber and a Li-6400 CO2/H2O IRGA. Seasonal variation regularities of root respiration rate were consistent with air temperature, moreover highest respiration rate observed in July and lowest rate in October. During the growth season, root respiration rate ranged from 0.5732 μmolCO2·g-1·s-1 (root ≤2 mm in diameter) to 7.1861 μmolCO2·g-1·s-1 (root ≤2 mm) on a dry weight basis or 0.0132 μmolCO2·cm-2·s-1(root ≤2 mm) to 0.6848 μmolCO2·cm-2·s-1(root >5 mm) on a surface area basis. Root respiration rate per unit dry weight decreased with increasing root diameter but root respiration rate per unit surface area had a reverse trend. Air temperature rising influenced the response of root respiration to nitrogen fertility, and the influence was significant level (P<0.05) between June and August. Root respiration rate increased exponentially with soil temperature at 10 cm (R2=0.779-0.981), with a Q10 of 3.45 to 8.48 for fine roots (root≤2mm in diameter) and 2.07 to 2.96 for thick roots (root> 2mm). Apparently, fine roots was more sensitive to temperature than thick roots. Nitrogen fertility influenced the Q10 of fine roots (P=0.0392<0.05) but not that of thick roots (P>0.05), thereby indicating that Q10 of fine roots was more easily affected by soil nitrogen concentration change than that of thick roots.