Abstract: Research on the production and function of reactive oxygen species (ROS) formed during early nodulation in legumes was reviewed. The ROS are highly reactive oxygen derivatives, including superoxide anion (·O₂^-), hydroxyl radical (·OH), hydroperoxyl radical (·HO₂), and hydrogen peroxide (H₂O₂). These radicals were produced during aerobic metabolism and when plants were under biotic and abiotic stresses. While toxic, these compounds serve as signalers that can trigger metabolism regulations to combat adverse environments. The ROS are produced during nodulation in legumes, induced by the nod factor (NF), to participate in infection formation, contributing to cell wall reconstruction, cross-linking of matrix glycoprotein (MGP), and actin microfilament nucleation and branching. Symbiotic production of ROS is primarily through cytoplasmic membrane NADPH respiratory burst oxidase homologue (RBOHs). Overexpression of Rboh stimulates rhizobia infection and nodule formation, resulting in increased number of symbiotic microsomes and nitrogen fixation efficiencies. Conversely, inhibition of Rboh decreases the production of ROS and down-regulates the expression of related RIPs, NIN, and ENOD2. The ROS regulate nodulation also in conjunction with Ca²⁺ through spatial and temporal alterations. In conclusion, ROS in legumes are a group of signaler molecules that function to regulate nodulation through genes expression.