Urmia Lake, the second largest hyper-saline lake by area in the world, has fluctuated in salinity over time, but recently, it has reached a maximum of 360 g/l. Dunaliella is a type of halophile green-orange microalgae especially found in lake and salty fields and is known for its antioxidant activity; because of its ability to create large amount of carotenoids. In the present study, Dunaliella sp. isolates from hyper-saline Urmia Lake water were cultured in modified Johnson media and were treated at different salinities (1, 2 and 3 M NaCl), then their cell proliferation rate and beta-carotene production were studied. In order to determine the optimal salinity required for the highest beta-carotene accumulation, cell count of Dunaliella sp. isolates; total carotenoids and concentration of the beta-carotene were determined by direct microscopic counting and spectrophotometry. In the samples with different salinities, the cell count and the beta-carotene content of Dunaliella sp. ranged between 0.53 and 2.21 x 10(6) cell.ml(-1) and 0.2 to 11.4 pg.cell(-1), respectively. At the end of the experiments, the maximum cell content mean and the highest beta-carotene content mean were obtained at 1 and 3 M NaCl concentrations, as 1.68 x 10(6) cell.ml(-1) and 8.94 pg.cell(-1), respectively.