this work aimed at determining the influence of roughage proportions and fat source of the diet on characteristics of carcass and comercial cuts of lambs. it was used 24 non-castrated santa inês male lambs, ad libitum fed diet with two proportions of roughage (30 and 70%) and fat (no fat, protected fat, and soybean) slaughtered at an avarage body weight of 35.4 kg (± 1.5 kg). animals fed 30% roughage diet showed the highest weights and carcass yields. the percentages of posterior arm and ham were higher in animals fed 30% roughage with no addition of fat source. total leg length and internal length were higher in animals fed 70% roughage diet while leg width was higher for those fed 30% roughage diet. addition of fat source in diets with high percentage of concentrate can increase carcass yields. this effect is higher when protected fat is used regarded to whole soybean. although diets do not have effect on most of these cuts, the effect on the ham confirms the influence of the diet on this noble cuts.