-3 highly unsaturated fatty acids (HUFAs), eicosapentaenoic (EPA) and docosahexaenoic acids (DHA), are particularly important in the physiological procedures of human beings and animals. However, human and animals including some marine fish species can not synthesize HUFAs from linoleic and linolenic acids since they have no or less activity of delta 6nor 5 fatty acid desaturases and elongase that are a key for the synthesis of HUFAs. Therefore, they have to ingest HUFAs from their diets for the normal growth and development. Actually, this is a question that should be further explored in some marine fishes. Cobia (Rachycentron canadum) is a worldwide marine fish in tropical, subtropical and warm temperate seas. Cobia is an excellent candidate for aquaculture because of its fitting to wide salty, high resistance to diseases, and rapid growth, reaching 6–10 kg in 12–14 months. Marine fish is main source of HUFAs, DHA and EPA for human beings. With the decline of fishing and rapidly expanding aquaculture, feed-grade fishing has reached sustainable limits. A developing cobia commercial fishery needs to solve sustainable developing problems of artificial nutrition feed, using plant products to replace fish products, in which the basis of nutrition, physiology and biochemistry of cobia is necessary to study. delta 6 fatty acid desaturase, a key enzyme in the synthesis of HUFAs in cobia was studied in our experiments. Delta 6 fatty acid desaturase cDNA of cobia () was cloned and sequenced as well as its expression in different tissues was determined. Total RNA was extracted from the liver of juvenile cobia and amplified using real time-PCR (RT-PCR). The amplified product was a fragment with 743bp. The sequence of the fragment of delta 6 fatty acid desaturase from cobia showed great identity (87%) with that of Dicentrarchus labrax). The protein sequence of the fragment included two transmembrane regions and two histidine boxes which were composite characteristics of a microsomal fatty acid desaturase. The enzyme gene expression in different tissues of cobia was determined using (RTQ-PCR) and their expressive orders were as follows: brain > liver > heart, intestine, spleen, kidney and gill > muscle and skin; while no expression was detected in adipose tissue. A conclusion is that cobia has delta 6 fatty acid desaturase, which is a key enzyme for the synthesis of HUFAs. The enzyme gene is maximally expressed in the brain of the fish sug