Abstract: This paper deals with the determination of the seasonal changes in the nodule nitrogen-fixation activity of eight species of plants in the chestnut soil zone in the Xilingoule grassland of Inner Mongolia. The results show that mainly due to the changes in temperature and moisture in the soil, the nodule nitrogen-fixing activity of the different plants show obvious seasonal changes. The highest takes place in summer, the average value being up to 529.6nmol·C2H4/min·g, and the lowest takes place in autumn, the value in spring being only about 10 percent of that in summer.Different plants have different characters of nitrogen-fixing activity. Medicago facata has the highest nitrogen-fixing activity among the eight species of plants. Trigonella ruthenica and Astragalus absurgens have rather low activity at their early stage, becoming relatively high later. Caragana mocrophylla and Hippophae rhamnoides have the lowest nitrogen-fixing activity among the eight species.