To evaluate the effects of various concentration of carbon dioxide and salinity stress on morphophysiological characteristics of Amaranthus tricolor L. an experiment was conducted in greenhouse conditions in Faculity of Agriculture Ferdowsi University of Mashhad. the experiment was split plot based on completely randomized design with three levels of CO2 (380, 700, 1050 µl/L) and three concentrations of sodium chloride (0,150, 300 mM) with 3 replications. The results showed that in control carbone dioxide (380 µl/L), application of salinity to 300 mM, reduced shoot dry weight, plant height and leaf area from 9.34, 53.83 and 1001 to 3.71, 35.3 and 158 respectively. Interaction effects of salinity and carbon dioxide showed that in control salinity and 1050 µl/L carbon dioxide, shoot dry weight and plant height increased 46 and 38% respectively. in level of 150 mM sodium chloride, with increasing concentrations of carbon dioxide from 380 to 700 µl/L, leaf area increased from 134 to 358 cm2. at this salinity level, in concentration of 1050 µl/L carbon dioxide, leaf area was 287 cm2. The final results showed that increasing concentrations of carbon dioxide improves adverse effects of salinity. in characteristics such as leaf area and electrolyte leakage, carbon dioxide in 700 µl/L and in characteristics such as root dry weight, shoot dry weight and plant height concentrations of 1050 µl/L carbon dioxide showed a better reaction to salinity.