Drought stress coincides with increasing atmospheric CO 2 concentrations is one of the most important challenges in crop production such as barley. Therefore, under these conditions it is necessary to identify the growth responses and related mechanisms of different barley cultivars. The present study was conducted to evaluate the physiological, biochemical and growth responses of 4 barley cultivar including: Gohran, Mehr, Behrokh, and M-90 under two irrigation regimes (40 and 75% of maximumallowabledepletioor deficit, MAD, as control and water deficit stress levels, respectively) as factorial in a completely randomized design with six replications. Also, two different environments in terms of carbon dioxide concentration (390 and 700 ?mol mol -1 ) was implemented in Open-top chamber(OTCs) of Isfahan University of Technology in 2018. The results showed that under enriched CO 2 condition, average grain weight decreased by 75% at water deficit (1.24 g plant -1 ) compared to normal irrigation level (2.17 g plant -1 ). However, under ambient CO 2 level, the average grain weight decreased by 50% at water deficit stress (0.9 g plant -1 ) compared to normal irrigation levels (1.35 g plant -1 ). With increasing carbon dioxide concentration, dry weight of barley cultivars increased by 48 and 14%, under normal and stress conditions, respectively. Elevated carbon dioxide had a significant positive effect on dry weight of Gohran, Mehr, Behrokh and M-90 cultivars and this effect was more under normal irrigation than water deficit stress. Under normal irrigation level, with increasing CO 2 , root dry weight (26%), plant leaf area (26%), plant height (18%), relative water content, chlorophyll a (28%), chlorophyll b ( 35%), malondialdehyde (MDA) concentration (22%), catalase activity (21%), ascorbate peroxidase (41%) increased but proline content (40%), guaiacol peroxidase (41%), and DPPH activity (49%), carotenoid (35%) decreased. However, under water deficit conditions with increasing carbon dioxide, root dry weight (84%), plant leaf area (6%), chlorophyll a (28%), carotenoid (88%), proline ( 32%), MDA (22%) and plant height (16%) increased catalase activity (21%), ascorbate peroxidase (47%), guaiacol peroxidase (39%) and DPPH activity (19%) and chlorophyll b (14%) decreased. As ageneral conclusion the amount and type of effect of elevated CO 2 on barley growth was soil moisture and cultivar dependent. Also, significant variation was observed among barley cultivars in terms of evaluated traits, which can be used to improve production efficiency under climate change. Keywords: Barley, Carbon dioxide, Chlorophyll a, Climate change, DPPH activity, Genetic variation