Osmotic stress promotes somatic embryogenesis of Fraxinus mandshurica , which leads to accumulation of reactive oxygen species (ROS). The single pieces of coty-ledons of F. mandshurica were used as explants to induce somatic embryogenesis in osmotic-stress medium. Further-more, the hydrogen peroxide (H2O2 ) content of explanted cells was varied by adding exogenous H2O2 or catalase solu-tion to assess the effects of the exogenous H2O2 on somatic embryogenesis, intracellular H2O2 accumulation, and the relationship between signaling mediated by ROS or reac-tive nitrogen species. The results revealed that exogenous H2O2 (100?300 μmol L –1 ) increased the number of somatic embryos. On 60th day of exogenous H2O2 (200 μmol L –1 ) treatment, the number of somatic embryos of explants treated, which was 136.54％, was higher than the control. Moreover, exogenous H2O2 (100 μmol L –1 ) significantly increased the intracellular H2O2 content and enhanced the activities of superoxidase dismutase and peroxidase. Finally, exogenous H2O2 (100 μmol L –1 ) activated the intracellu-lar non-enzymatic pathway for nitric oxide (NO) synthesis. The somatic embryogenesis in broadleaf trees increases with the change of endogenic ROS content, and depends on the upregulation of antioxidant enzymes. Both H2O2 and NO, as signaling molecules, were found to be involved in the pro-cess of somatic embryogenesis in broadleaf trees. In the pro-cess of exogenous H2O2 promoting somatic embryogenesis, NO synthesis depended on non-enzymatic reactions. These results provide a scientific basis for resolving the mechanism by which ROS levels are regulated during somatic embryo-genesis of broadleaf trees and establish a reasonable and efficient technology system for regulating somatic embryo-genesis of trees.