A POT experiment was conducted in the greenhouse of the National Research Center, Dokki, Cairo, Egypt during winter season of 2014/2015 to evaluate the effect of salt stress from diluted seawater (tap water, 2000, 4000 and 6000ppm) on growth, yield and chemical composition of three onion varieties (Giza 6, Giza 20 and Behary red). Nether top height nor the number of green leaves cleared any significant difference between the used three varieties. Furthermore, the bulb/top ratio or bulb/whole plant ratio of Giza 6 plants exceeded those of Giza 20 or Behary red. Behary red leaves showed the lowest values of Chl. a, Chl. b, total carotenoids and Chl. a+Chl. b. However, the highest values of these parameters were detected in the leaves of plants of Giza 20 variety and those for Giza 6 comes in between. Top height decreased but the number of green leaves did not affect by salt stress. However, the top and whole plant dry weight drastically depressed by increase salt concentration in water of irrigation (diluted seawater). Nevertheless, bulb to top ratio and bulb to whole plant ratio (in dry weight basis) increased as the salt level increased in the root media. The results also indicated that salinity promoting the concentration of these pigments and this promotion increased until 4000ppm salt level and tended to decrease with the highest level used but still more than the control.