Molybdenum is an essential microelement for the symbiotic growth of soybean plants. With treatments of molybdenum, soybeans became taller, leaves expanded larger and the leaves contained more chlorophyll.Inaddition,thesizesandnumbersofnodulcs’alsoincreased. However, the fluctuations of nitrogen fixing activity of root nodules in the growing season could not entirely be interpreted by these two parameters. For example, plants which were treated with molybdenum at pre-pod filling stage, had heavier nodules than those ones treated with molybdenum at pre-flowering stage, however, the latter plants had higher nitrogen fixing activity than the former ones. The leghaeraoglobin content in root nodules did not change as much by the molybdenum treatments as the nitrogen fixing activity did. This means that leghaemoglobin content in root nodules could not be considered as a criteria for judging the activity of nitrogen fixation of plants. Although four components of leghaemoglobin were separated by gel electrophoresis, these electrophoretic components of leghaemoglobin did not vary with the treatments of molybdenum. This suggests that all of these four components of leghaemoglobin may be present simultaneously carrying oxygen to the nitrogen fixing rhizobia. The nitrogen fixing activity of plants was remarkably stimulated by molybdenum treatments, because molybdenum enhances the nitrogenase activity and promotes the photosynthetic capacity of plants. The increments of nitrogen fixing activity induced the soybeans to form more numerous pods and seeds, consequently, the overall productivity of the plants was increased, and plants receiving molybdenum throught the roots give better yields than those receiving molybdenum through the leaves. The proper time of supplying molybdenum to plants is at the pre-lowering stage.