Abstract: Seeds of Medicago sativa were used to study the effects of 0.1 mmol/L sodium nitroprusside (SNP), as a source of nitric oxide, on seed germination and antioxidant capacity under imbibitional chilling at 4℃. Four treatments: control, chilling, SNP and chilling+SNP were replicated three times. The results indicated that after imbibitional chilling for 12 h and germination at 25℃ for 84 h, the vigor and germination of M. sativa seeds were increased by SNP treatment. The activities of antioxidant enzymes including superoxide dismutase (SOD) and catalase (CAT) increased 149.3% to 60.4% by SNP treatment. The radical clearance rate tended to decline with SNP treatments and chilling for 12, 24 and 48 h. At 12 h chilling the radical clearance rate reached a maximum of 97.5%. Imposing exogenous nitric oxide can promote germination, enhance antioxidant enzyme activities and reduce the reactive oxygen damage in M. sativa seed.