Eighteen Saidi rams were used in this trial to study the impact of additive mannan oligosaccharide (MOS;activeMOS®) on some of carcass characteristics. MOS are commercially available as BioMos®, which is a nutritional supplement manufactured by MOS ® Matrix nutrition, LLC, USA was used in this experiment. Animals were randomly divided into three equal groups. The initial average live body weight values were 24.00,24.08 and 24.17 kg for groups 1,2, and 3 respectively. The first group did not receive MOS and served as a control group, while the second and third groups were supplemented with 2 and 4 g/ kg diet MOS and served as a MOS1 and MOS2 groups, respectively. Both experimental groups were fed roughage and concentrate diets ad libitum during this study. At the end of the experimental period, lasted for 6 months, final average body weight values were 44.17,48.50 and 45.83, respectively. Five animals from experimental groups were slaughtered. The data revealed that supplementation of dietary MOS in the diet of siadi rams increased hot carcass weight, dressing percentage, right (P<0.05) and left side percentages. Moreover, most of edible and non edible parts tended to be heavier for MOS treated rams, while fat of heart, kidney and pelvic, gut, intestine and total fat tended to be lower for these groups than a control group. The proportion of muscle/ bone and muscle/fat ratios increased in treated groups. Left carcass side weight and carcass cuts were heavier in animals fed diet supplemented with MOS than controls. Meanwhile, a high priced cuts (leg, sirloin and best neck and fillet) were heavier by 29.05% and 12.7% of rams fed diet supplemented with MOS1 and MOS2 additives ,respectively compared with the control. The highest part of high priced cuts was observed in fillet cut for MOS1 and MOS2 supplementation by about 57.14% (P<0.05) and 14.29%, respectively than control rams. Individual skeletal muscle of Semimemberanosus (SM), Supraspinatus (SP) and Longissimus dorsi (LD) were increased in the case of the dietary supplementation with MOS. Consequently, it appears from the present study that the dietary of MOS improve carcass characteristics and meat quality. Moreover, MOS inclusion at 0.2% was the most effective, suggesting that MOS might be a potential type of food additive useful for the growing sheep in Upper Egypt conditions.