THIS STUDY was conducted to investigate the effect of NaCl on ……the physiological and biochemical traits of Chlorella vulgaris. The Alga was exposed to different concentrations of NaCl ranging from 50-300 mM besides control over a period of 10-20 days. Total chlorophyll contents and carotenoids levels were increased at low NaCl concentrations but significantly reduced at higher concentrations. It is interesting to note that total free amino acids and proline contents increased at low and moderate NaCl concentrations. The activity of the antioxidant enzymes; CAT, POD, PPO, and SOD; were noticeably increased by increasing salt concentration up to 200 mM and thereafter declined. The photosynthetic related genes expression; rbcL, psaB, and psbC; were significantly reduced at all NaCl concentrations. The results indicated that salt stress inhibits PSII efficiency and reduces the overall CO2 assimilation rate in Chlorella vulgaris