Although high concentrate diets are commonly used for finishing lambs, literature about the effect of minimum levels of NDF on performance is limited. The purpose of this study was to evaluate the effects of increasing dietary fiber levels on performance and carcass parameters of in finishing lambs fed high concentrate diet. Twenty-four male lambs (initial BW of 24 ± 3 kg, 90 d old) were randomly assigned to 24 individual pens, in a complete randomized design with 3 treatments and 8 replicates. Treatments were dietary NDF levels of 12, 15 and 18% on DM basis. Diets were isoproteic, composed of corn, soybean meal, minerals and corn silage, and were offered twice a day with refusals daily measured to determine dry matter intake (DMI). Animals were weighed weekly and slaughtered after 50 d of feedlot, with average BW of 40 ± 4 kg. Hot and cold carcass weight (HCW and CCW) as well as hot and cold carcass yield (HCY and CCY) were measured. Data were submitted to polynomial regression analysis, with initial BW as a covariate, using the statistical software R. There was a linear increase in DMI (P < 0.01) from 1.26 ± 0.13 to 1.47 ± 0.21 kg/animal/d and a linear decrease on residual feed intake (P < 0.01) from 0.09 ± 0.11 kg/d to −0.06 ± 0.19 kg/d as dietary NDF increased. There was no difference in ADG (P > 0.05, 0.320 ± 0.06 kg/d) and feed conversion (P > 0.05, 4.27 ± 0.64 kg/kg) among treatments. There was a linear decrease (P < 0.05) on HCW (21.08 ± 1.10 to 19.65 ± 2.20 kg), HCY (51.93 ± 1.99 to 49.71 ± 2.47%) and CCY (50.04 ± 2.02 to 47.98 ± 2.64%) as dietary NDF increased. There was a trend of linear decrease (P = 0.06) for CCW from 20.31 ± 1.08 to 18.97 ± 2.15 kg as dietary NDF increased. In conclusion, although ADG was not affected by treatments, carcass weight and carcass yield increased as dietary levels of NDF decreased.