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Recently, a great deal of interest has been developed to isolate novel bioactive compounds from natural resources due to their numerous health beneficial effects. Among them, the polysaccharides are widely distributed in the nature and their molecular structures have caused to their various biological activities. Chitosan is a naturally abundant β-1,4-linked copolymer of N-acetyl-2-amino-2-deoxy-D-glucopyranose and 2-amino-2-deoxy-Dglucopyranose units and is a naturally occuring, biodegradable, non-allergenic deacetylated derivative of chitin, found abundantly in nature. Even though chitosan is known to have important functional activities, poor solubility makes them difficult to use in the food and biomedicinal applications. Unlike chitosan, its hydrolyzed products as chitosan oligosaccharides (COS) are readily soluble in water due to their shorter chain lengths and free amino groups in D-glucosamine units. The bioactive properties of chitosan and COS depend on their degrees of deacetylation and molecular weight. Until now, there are very less information available on bioactive properties based on both degrees of deacetylation and their molecular size. It has been reported that novel COS derivatives can be used as physiologically bioactive materials since they possess various health beneficial biological activities, such as antioxidative, antitumor, antimicrobial, anti-hypertensive, antidiabetic, calcium absorption enhancing hypo-cholesterolemic activities and immune enhancing effects. Therefore, it is needful to prepare various types of COS derivatives, such as N-acetyl COS, Amino ethyl-COS (AE-COS), Demethyl amino ethyl-COS (DMAE-COs) and sulfated COS with different degrees of deacetylation and molecular weight. furthermore, the determination of their uninvestigated biological activities such as enzyme inhibitory (β-secretase, MMP, AchE), intra-cellular free radical scavenging, and anti-HIV activities are useful to expand the health of human being. The use of these novel COS derivatives as nutraceuticals, functional foods, pharmaceuticals and cosmeceutical promissing.