Abstract: Medicago truncatula seeds were pre-soaked with 0.1, 0.3 and 1.0 mmol/L sodium nitriprusside [SNP, a nitric oxide (NO) donor] solution to study the effects of nitric oxide on seed germination, physiological characteristics and active oxygen metabolism of M. truncatula under 2.0% NaCl stress. Damage to M. truncatula seeds caused by 2.0% NaCl stress was significantly alleviated in the presence of 0.1 mmol/L SNP. The seed germination percentage, germination energy, germination index and vigor index were increased 333.40%,79.64%,171.93% and 100.00%, respectively; while soluble sugar, protein and proline contents in seeds were increased by 21.1%, 42.3% and 123.1%, respectively. Starch content was decreased by 17.7%, amylase activity was increased by 29.7%, and MDA and O2-· contents were decreased by 21.8% and 27.2%, respectively. Activities of SOD, POD, CAT and APX in seeds were increased by 28.6%, 43.1%, 56.2% and 60.3%, respectively. Seed germination, oxidative damage and the antioxidant enzyme activities in seeds decreased when seeds were treated with 0.3 mmol/L SNP. When the concentration of SNP solution was increased to 1.0 mmol/L, we observed an inhibitory effect on seed germination. Our results provide evidence that lower concentrations of SNP may alleviate damage to seeds caused by salt stress. Key components of the response are: decreased MDA and O2-· levels, increased proline level and antioxidant enzyme activities in seeds, with resultant promotion of starch and protein hydrolysis, expedition of seed germination.