This research was conducted to determine the effects of supplementation of chitosan oligosaccharide (COS) on the growth performance, blood parameters, carcass traits, fatty acid composition of breast meat, and apparent nutrient digestibility in broiler chickens. A total of 375 one-day-old Ross 308 male chicks were allocated to a control diet (T1) or diets supplemented with 50 ppm (T2) and 100 ppm (T3) of COS. There were five replicates of 25 chicks for each treatment. All the experimental birds were fed a starter (days 1 - 4), grower (days 15 - 28), and finisher diet (days 29 - 42). No differences were detected among treatments for live weight, gain, feed intake or feed conversion, except that feed intake was depressed in T3 during the grower period. Serum total protein and albumin levels did not differ among the treatments. Serum total cholesterol, low density lipoproteins (LDL), very low density lipoproteins (VLDL) and triglyceride concentrations were reduced significantly by supplementation with COS, whereas the high density lipoprotein (HDL) concentration was significantly lower only for T2 relative to T1. The dressing percentage was significantly higher for T2 and T3 than for T1. Fatty acid composition of the breast meat was unaffected by the treatments. The digestibility of the diet and some of its constituents was affected in a graduated manner by the addition of COS. Thus, supplementation of broiler diets with COS improved carcass yield and had a hypolipidemic effect in improving the serum lipid profile
 Keywords: carcass, digestibility, hypolipidemic effect, performance