In order to evaluate the effect of sodium nitroprusside (SNP) as an effective antioxidant with advantageous impact in reducing oxidative stress in purple coneflower (Echinacea purpurea) seedlings under saline condition, a factorial experiment with arrange of completely randomized design with four replications was conducted at Sana Institute of Higher Education in 2015. In this study, the effect of the SNP as doner of nitric oxide (NO) at 4 levels ( 0, 0.1, 0.2 and 0.4 mM) on some biochemical characteristics of purple coneflower under 3 levels of salinity stress (0, 75 and 150 mM of NaCl solution) were evaluated. The results showed that the interaction between SNP and salinity on fresh and dry weight of root and leaf, carotenoid and malondialdehyde (MDA) content, enzyme activities of superoxide dismutase (SOD), peroxidase (POD) and ascorbate peroxidase (APX) were significant. With increasing of salinity concentration, root fresh and dry weights were decreased, while the traits were increased by 0.2 mM of SNP under salinity. The highest of leaf fresh and dry weight were observed in the control treatment. The chlorophyll a, b and total chlorophyll contents were decreased with increasing of salinity, while the highest amount of chlorophyll were measured 0.4 mM SNP under normal conditions. The highest level of total phenol was observed in the highest level of salinity and a higher level of MDA was also observed in control plants under salinity stress. Protein content was also decreased 5.4 % under salinity stress. The results also showed that 0.2 mM of SNP pretreatment was increased activities of antioxidant enzymes SOD, POD and APX in plants under the stress