Abstract: The discovery of nitric oxide (NO) established a complete new concept on the cell signal transduction, i. e. the gas signal produced from one cell can pass through the cell membrane to regulate the functions of another cell. NO has unique physical and chemical natures. NO in vivo is synthesized by L-arginine and oxygen under the action of NO synthase (NOS). NOS mainly includes 3 kinds of subsets: endothelial NOS (eNOS), neuronal NOS (nNOS) and inducible NO S (iNOS). These NOSs are regulated by Ca²⁺/calmodulin, phosphorylation and NO. The biological functions of NO are very wide and bidirect. NO might facilitate the radiosensitivity of cells in vitro, but display a radioprotective effect in vivo.