Starch is the major source of energy in the diets of feedlot cattle, comprising 50% or more of the dietary DM. With limitations to starch digestion in the small intestine and low energetic efficiency of starch fermentation in the large intestine, it is important to maximize ruminal starch digestibility, while concurrently minimizing digestive disorders such as acidosis. Grain processing is the most common strategy to increase ruminal digestion of starch. Supplemental amylase has been proposed as a means of improving total tract starch digestion, but research on the effects of exogenous amylase in feedlot diets is limited. To evaluate addition of amylase to feedlot diets, 32 Angus and Angus-crossbred steers were used in a randomized complete block design with replication of treatments in the block in a 2×2 factorial arrangement. Factors included grain processing method (dry-rolled corn [DRC] or steam-flaked corn [SFC]) with or without α-amylase (RumiStar; DSM Nutritional Products, Inc., Kaiseraugst, Switzerland) supplemented at 600kilo novo units (KNU)/kg of dietary DM. One KNU is the amount of enzyme that releases 6μmol of p-nitrophenol per min at pH 7.0 and 37°C. Steers gradually adapted a 90% concentrate finishing diet for 28d, during which the DRC and SFC treatments were applied. Amylase was supplemented when steers reached ad libitum intake of the 90% concentrate diet (d 1 of experimental period). Experimental diets were fed for 42d, and 0.25% (DM basis) of Cr2O3 was included in the diet for 11d as an indigestible marker for measurement of nutrient digestibility from d 39 to 42. Supplementation with amylase did not affect nutrient digestibility (P≥0.21) or DMI, ADG, and G:F for the 42-d period (P≥0.35). The SFC diets had 28% less total tract digestibility of NDF (P<0.001) and 39% less total tract digestibility of ADF (P<0.001) than DRC-based diets. Total tract starch digestibility was increased by 5.5% (P<0.001) in SFC-based diets. Steers fed the SFC-based diets had lower DMI (P=0.04) and increased ADG (P=0.04) and G:F (P<0.001) compared with steers fed DRC-based diets. Overall, results suggest that supplementation with exogenous α-amylase at 600KNU/kg dietary DM did not affect nutrient digestibility or performance by feedlot steers. Steam flaking corn grain decreased total tract digestibility of NDF and ADF and increased total tract starch digestibility compared with dry rolling, which was associated with greater ADG and G:F.