A balanced supply of nutrients can be regarded a way to support an increase of body weight and a tool to improve the carcass grade at slaughter. In that context carcass weight, yield and fat covering can be changed by the quality of different diets affecting not only the carcass parameters but also the meat value of the lambs. The carcass grade and weight at slaughtering and its parameters were evaluated on feedlot lambs fed different sources of non-protein nitrogen (NPN) and carbohydrates (CHO). It was used 20 non-castrated lambs with 4 mo of age and 26 kg of body weight. Lambs were fed ad libitum for 45 d. The diets were composed by 57% of grass hay (Brachiaria hybrid) and 43% of concentrate. The hay was harvested at late mature stage with high (72.4%) neutral detergent fiber and low (4.9%) crude protein. Four concentrates were formulated from the combination of 2 CHO (ground corn: starch; whey permeate: lactose) and 2 NPN (fast and slow release urea) sources. Slaughter weight (SW), body condition score (BCS), hot carcass weight (HCW), cold carcass weight (CCW), fat thickness (FT), hot carcass yield (HCY), cold carcass yield (CCY) and chilling losses (CL) were measured. A completely randomized design was used with 4 diets and 5 replicates. Data were analyzed by ANOVA following a 2 × 2 factorial scheme. SW was higher (P < 0.05) for lambs fed corn (31.3 kg) than for those fed whey (29.4 kg) suggesting that the starch intake was more effective in improving low quality forage degradation than lactose. However, BCS (3.0 ± 0.1), HCW (12.34 ± 0.41 kg), CCW (11.87 ± 0.37 kg), HCY (40.7 ± 0.5%), CCY (39.2 ± 0.5%), FT (1.23 ± 0.10 mm) were not influenced (P > 0.05) by diets. CL was higher (P < 0.05) for diets with slow release urea (4.01%) compared with those with fast release urea (3.65%) suggesting higher muscle water retention for diets with urea of faster degradation. For this reason, we can recommend the use of a concentrate diet composed of fast release urea and corn for finishing lambs. Because these feed ingredients are usually cheaper, this will also reduce the cost of fattening.