Salinity is one of the most important plant stresses affecting plant growth. This study was carried out to investigate the effect of melatonin on increasing the resistance of sweet pepper to salt stress in a factorial experiment in according to a completely randomized design with three replications and each replication consisting 3 plants. Salinity treatments were applied at three levels including control (normal water), 100 mM and 200 mM sodium chloride solutions, and melatonin were spread at four levels including control (distilled water), 50 ?M, i100 ?M and 150 ?M of melatonin. Salinity had a significant effect on all traits except for root water content. Length, fresh and dry weight of shoots and roots, greenness index, efficiency of quantum system II (Fv / Fm), photosynthesis rate, trairation rate, stomatal conductance, mesophylic conductance, photosynthetic efficiency and protein content decreased and shoot water content, stomatal carbon dioxide, ion leaching percentage, leaf proline content, antioxidant enzymes of catalase and peroxidase, and antioxidant capacity increased under salinity stress condition. At both levels of salinity, melatonin increased electrolyte leakage and proline levels. At 100 mM salinity stress, melatonin significantly increased shoot length at all concentrations, but at 100 and 150 ?m, it reduced shoot water content and root fresh weight. Also, melatonin significantly reduced root dry weight, photosynthesis and leaf stomatal conductance at 150 ?M and leaf protein at 50 and 150 ?m concentrations. At 200 mM salinity, melatonin spray significantly decreased the greenness index and Fv/Fm at all concentrations and increased peroxidase activity at 100 ?m concentration. The results showed that salinity generally reduced vegetative characteristics, photosynthetic parameters and protein content but increased ion leakage, proline, enzymes activity and total antioxidant capacity. Although application of melatonin in salt stress conditions, may have a positive effects on shoot length and leaf proline, but it had a negative effect on growth parameters such as root fresh and dry weight, photosynthetic parameters, leaf greenness index, Fv / Fm and significantly reduced the amount of these parameters. However, under normal conditions (without stress), melatonin had positive effects in many cases such as on the root length, root fresh weight, photosynthesis, and stomatal conductance, and in a few cases, negative effects was observed. Generally, in this experiment, melatonin application under salt stress, similar to stress treatment had negative effect on pepper seedlings and increasing activity of antioxidant enzymes, increase in electrolyte leakage and proline along with growth retardation and decrease in photosynthetic parameters, Fv/Fm and leaf greenness index may confirm this claim.