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In this survey, hydrology and hydrobiologycal studies in the northern part of the Oman Sea and Strait of Hormuz within the Iranian waters were conducted during 2007 and 2009.The project was implemented using Ferdous research vessel.The sampling area included the whole stretch of the Iranian waters along the northern part of the Oman Sea from the northeast of the Iranian marine border of 30 miles Bay of Chabahar to the mouth of the Hormuz Strait in Hormuzgan province. Along this stretch 10 equally distributed transects at a distance of 30 miles away from each other were designated, 4 main sampling stations at a distance of 10 miles from one another were fixed at each transect. Sampling was conducted twice a year (spring and autumn 2007 and 2009). According to achieved results; we can point out to the following outlines below: permanent thermocline exists annually in this body of water; where only the point of start, and the fracture width of thermocline layer; alter with alteration of seasons. Horizontal and vertical distribution of electrical conductivities, obey exactly from thermal degree structure.Level of salinity increase from east to west, but it decrease; from surface to deeper layers. In deeper regions, especially in the midway east of the Oman Sea; the level of salinity in the deeper waters from 150-300m, there is a considerable increase observed in salinity, due to very high salinity waters of The Persian Gulf; that are entering the Sea of Oman, via the Striate of Hormouz from the bottom; where they are having their effect following the whole length of the Sea of Oman; through their path. The level of water density from offshore to onshore, and from surface to deeper levels increase in such a way that. The level of Chlorophyll-a in surface waters, in the northeastern midway; is higher than the northwestern midway of the Sea of Oman, and it decreases from inshore to offshore waters, but its vertical distribution; has caused somehow in a way, that the highest distribution and concentration of chlorophyll-a; to be formed in deeper layers, between about 10-40m depths. Then by the increase in water depth, its concentration decreases drastically; reaching zero in magnitude. Concentration of dissolved oxygen is higher in spring in comparison to that of autumn, and the highest level is achieved in water layers located at 10-40m depth (where the level of chlorophyll-a is higher). In addition to which, that its vertical structure; shows the existence of a permanent oxycline layer at this region, in a way that; by seasonal alterations (similar to that of thermocline layer), only the point of start and that of the thickness of oxycline layer changes.pH level decreased from water surface, and its vertical alteration trend; obeyed to that of the same for temperature and chlorophyll-a vertical structure, especially for dissolved oxygen parameter, where the maximum decrease was recorded coinciding with formation of oxycline layer.Nutrient levels increased; at surface water layers from offshore to inshore, and from west to east, for the same body of water. In addition to that, concentration of nutrients in autumn (after monsoon); were more than that of, spring season (before monsoon), where; concentration of their vertical structure increase from water surface toward deeper sections. In this survey six phylum of phytoplankton including in Bacillariophyta (88) Pyrophyta (111), Cyanophyta (6), Chropmophyta (2), Euglenaphyta and silicoflagellate (1) were identified. A result showed that density decreased in most transects from inshore toward sea and maximum density was in photic layer (0-25m). Phytoplankton densities (without Cochlodinium density) in post-monsoon were recorded higher than pre-monsoon, furthermore phytoplankton density in 2009 was more than 2007(6073±1038) Species richness was indexed in the pre-monsoon and post-monsoon 2007 (0.337-0.519), (0.296-0.396) respectively and to (0.967-1.525), (1.407-1.531) for 2009. Zooplankton population in this study is characterized by eight phyla, eight classes, 15 orders, 35 families and 78 genus. In 2007, the pre-monsoon percentage of groups were frequently as following: Copepoda with stages of Nauplius and Copepeditide were 78%, Ciliphora 8%, Sarcomastigophora 4% and Hemichordata 6%. In post-monsoon order of frequency Copepoda 80%, Ciliophora 6%, Coelenterata 5%. Copepoda was the most abundant group of zooplankton.In 2009, The pre-monsoon Copepoda (82%), Cilillophora(9%), Sacromastigophora(4%) and Hemichordata (2%) in post-monsoon Copepoda(70%), Ciliophora (20%),Sarcomastigophora(4%) and Hemichordat (2%) dominante group were respectively. The static result showed significance betweendifferent transects and layers.We were recorded 31 ichthyoplankton families with different density and distribution during 2 years. Among benthic invertebrate polychaete with average 800 ind-2 and then amphipods and gastropods were the most abundance.Yearly density of macrobenthos showed polychaete with the average of 500 ind.-2 in 2007 and 1000 ind.-2 in 2009 was the most abundance . Macrobenthos density in postmonsoon was more than premonsoon.p0.05.There was a negative regression between macrobentos density and depth(p