The effects of arbuscular mycorrhizal fungi (AMF), Glomus mosseae, on oxygen radical scavenging system (including superoxide dismutase (SOD), catalase (CAT), glutathione reductase (GR, ascorbate peroxidase (APX) and peroxidase (POX)) in nodules of soybean (Glycine max) plants under salt stress condition were studied in potted culture experiment. The experiment was arranged as a factorial in Randomized Complete Block Design (RCBD) with four replications in greenhouse of College of Agriculture, Tehran University, Iran. Results indicated that AMF colonization notably increased the activities of SOD, CAT, POX and GR in the nodules, whereas it had little effect on APX. The results indicate that the AM fungus is capable of alleviating the damage caused by salt stress on symbiotic nitrogen fixation of soybean plants by increasing antioxidant enzyme activity. In conclusion, AMF could enhance the salinity tolerance of soybean plant, and thereby play a very important role in improving symbiotic nitrogen fixation and promoted plant growth.