Abstract: Alnus mandshurica and A. sibirica var. hirsuta are tastgrowing species with ability of N₂-fixation widely distpihuted in northeast in China. Assay of N₂-fixing activity and carbon cost by nodule of the two species was conducted in field and laboratory conditions. Techniques of acetylene reduction and infrared gas analyzer were used in the experiments.Results show: 1) Soil temperature was the major environmental factor affecting N₂-fixing activity of alder nodule. 2. The optimum temperature in N₂-fixation was 20℃ for A. mandshurica and 25℃ for A. sibirica var. hirsuta. 3. Ratio of value of N₂ fixed to CO₂ evolved was maximum at 20℃ for A. mandshurica and 25℃ for A. sibirica var. hirsuta. Efficiency of N₂-fixation at temperature bellow 20℃ was higher for A. mandshurica than for A. sibirica var. hirsuta. But it was reverse at the temperature above 25℃.Measures for increasing soil temperature of site are useful in extending the potential ability of N₂-fixation of alder.