Abstract: The aim of this study was to evaluate the relationship between rumen fermentation parameters and rumen papillae characteristics in finishing Holstein bulls fed three levels of Nonfibrous Carbohydrates (NFC) and two lipid sources. Thirty-four male Holstein bulls were randomly assigned to six different concentrates with three levels of NFC: 450 (low), 550 (medium) and 650 g per kg of DM (high) and two lipid sources (whole canola seed vs whole linseed). Corn meal was the main source of NFC in all treatments. Concentrates were isonitrogenous and isocaloric. Animals (initial weight = 300±25.5 kg) were fed barley straw and concentrate ad libitum until slaughter weight (443±12.5 kg). Animal body weight was recorded every two weeks and feed consumption weekly. To study rumen fermentation parameters, a rumenocentesis was performed monthly. At slaughterhouse, samples from the rumen wall mucosa of the ventral floor of the cranial sac were collected. After separating rumen papillae individually, a digital vision technique was used to estimate papillae numbers per cm2, average papillae surface area (mm2) and mucosa surface area (mm2/cm2). Daily concentrate and straw intakes were greater in animals fed canola seed than in animals fed linseed. In contrast, lipid source did not affect rumen parameters and papillae characteristics. Daily concentrate intake was lower in animals fed low NFC (6.93 kg DM/d) than in those fed medium NFC (7.21 kg DM/d) and high NFC (7.15 kg DM/d). Rumen molar concentrations (mM) of Volatile Fatty Acid (VFA) decreased significantly in animals fed the low NFC diet and consequently, rumen pH tended to be greater in these animals (6.53) compared with those the fed medium NFC (6.24) or the high NFC (6.28) levels. Animals fed the medium and the high NFC levels had greater propionate and n-butyrate rumen molar concentrations and tended to have a greater average papillae surface area and mucosa surface area than those fed the low NFC level. Regression equations showed a quadratic relationship between average papillae surface area and rumen propionate molar concentration (R² = 0.18). Number of papillae (R²=0.30) and mucosa surface area (R²=0.22) increased quadratically with rumen propionate molar concentration.