Abstract The present work aims to study the potentialities of macroalgae Gelidium sp. collect from Mediterranean Sea as new source of bioactive substance. Thus, the chemical and biochemical components for these algae were isolated and characterized which includes antioxidant activity. The proximate chemical composition of Gelidium sp. were protein content (13.23 ± 1.06% DW), crude lipid (1.16 ± 0.21% DW), fiber content (5.5 ± 1.05% DW), ash content (26.45 ± 0.74%), and carbohydrate content (53.66 ± 1.21% DW). The fatty acid profiles of Gelidium sp. were analyzed as follow, the saturated fatty acids are caproic, caprylic, capric, lauric, myristic, pentadecanoic, palmitic, heptadecanoic, stearic and arachidic acid, and the unsaturated fatty acids are tetradecenoic, oleic, vaccinic, octadecosaenoic, linoleic, linolenic and alpha octadecatetraenoic acid. Most of the essential amino acids found in Gelidium sp., which accounted for 50.26% of the total amino acid, glutamic and aspartic acids are the most abundant amino acids in Gelidium sp. (24.41 % of total amino acids). Polysaccharides from Gelidium sp. were extracted and the yield was (15.2%), Analyses of total sugars by the method of DNS colorimetric showed high percentages of these compounds (82.9%). The monosaccharide composition of galactans obtained from Gelidium sp by reductive hydrolysis and quantified by gas chromatography, showed that these galactans have a high content of galactose (83.4%) and 6-O-Me-galactose (14.1%) where glucose was found in smaller quantity (2.5%). DPPH is a free-radical compound that has been widely used to determine the free-radical scavenging ability of samples; result demonstrated that the sulfated polysaccharide had a noticeable effect on inhibiting the formation of these radicals (68%). Where the result obtained for the inhibition of hydroxyl (OH–) radical formation demonstrated that scavenging activity of Gelidium sp. polysaccharide was (57%).