In oceanography study, factors of temperature and salinity of water are very important, specially for ecosystems at estuaries that dominated by tide. By using CTD equipment, the general picture of temperature - salinity vertical structure of water is displayed clearly in Hai Phong coastal area. In the Eastern part of Do Son, gradient of water temperature was 0.1oC/m; the salinity of water was stable in the depth of 3.5m to the bottom (6.5m). In the Northern - West of Cat Ba Island, the water temperature in vertical structure at P-7 point was stable in the surface layer to 0.7m deep and strong disorder from 0.8m deep to bottom (2.8 m). At P-9 point, water salinity was vertically quite stable and ranged from 27.5‰ to 27.8‰.