Abstract: Artemia is a branchiopod crustacean that inhabits hypersaline habitats and has the ability to produce dormant embryo in gastrula state with tertiary membrane that named cyst. The main source of Artemia in Iran is Urmia Lake. The present study under taken to compare ultra structure of shell in 2 different groups Artemia urmiana cysts with different ability in buoyancy and role of shell layer in buoyancy or sinking. Cysts of Artemia urmiana collected from 2 levels of water (surface and bed) and for electromicroscopy fixed in glutaraldehyde and osmium tetroxide and dehydrated by graded series of ethanol and propylene oxide. Then, they embedded in resin (Agar 100) and 70-80 nm sections colored in 2 stages by uranyl acetate and lead citrate. Ultra structural observation showed some differences in thickness of alveolar (4.5-16 �) and fibrous (5.5-0.65 �) layers. Results as indicated from structural differences, showed shell layers are effective in buoyancy or sinking of cysts and against previous ideas about non-relation between buoyancy and shell layers thickness, confirm that cysts set in different depths based on their thickness specially alveolar layer.