In feedlot system is particularly important to reduce the cattle feeding cost without impact on the animal gain and carcass, in this sense, aimed with this study to evaluate animal performance and carcass traits of the young Nellore male (n = 40) finished with agro-industrial by-products in feedlot diet. The addition of cottonseed by-product (CSB) was based on the ether extract (EE) contents in the feedlot diet: 3, 4 and 5%; and two other reference treatments were also tested, with 3 and 5% of EE content and soybean by-product (SOB) as fat source, totalling five experimental diets. In diets with 3% EE, CSB did not alter performance, gain cost or carcass traits compared to the SOB. In diets with 5% EE, animals fed with CSB showed greater dry matter (DM) intake than animals fed with SOB (10.34 versus 8.94 kg/day), but because CSB is a cheaper ingredient than SOB, it reduced the gain cost from 1.60 to 1.35 US$/kg. The CBS used in diet with 3, 4 and 5% EE increased the daily gain (1.17, 1.38 and 1.50 kg/day) and the rumination time (225, 338 and 370 min/day, respectively). So, CSB does not change the carcass traits nor the feeding behaviour when compared to SOB. The increased of CSB concentration in the diet raised the daily gain, DM intake and rumination time, with no changes in carcass traits.Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Universidade Federal do Mato Grosso (UFMT), Instituto de Ciências da Saúde, Sinop, MT, BrasilUniversidade Estadual Paulista Júlio de Mesquita Filho (UNESP), Faculdade de Ciências Agronômicas (FCA), Departamento de Economia, Sociologia e Tecnologia, Botucatu, SP, BrasilUniversidade Estadual Paulista Júlio de Mesquita Filho (UNESP), Faculdade de Ciências Agronômicas (FCA), Departamento de Economia, Sociologia e Tecnologia, Botucatu, SP, Brasil