This experiment was conducted to study the effects of five different ratios of barley grain to maize grain: 1) 100:00, 2) 75:25, 3) 50:50, 4) 25:75, and 5) 00:100 on feedlot performance, carcass quality, ruminal and blood parameters of twenty-five Holstein bulls. Quadratic effects were detected for carcass efficiency and were greatest for diets with barley grain-to-maize grain ratios of 75:25 and 50:50 compared with other diets. Also, back-fat thickness, abdominal fat, and meat ether extract increased as the proportion of maize increased in the diets (P < 0.05). When the proportion of maize in diets increased, the amount of undigested grain in feces increased linearly (P < 0.05). At 3 and 6 h after feeding, as the proportion of barley increased in the diets, ruminal pH decreased quadratically. We conclude that feeding bulls diets based on 50:50 and 75:25 barley-to-maize grain ratios resulted in higher carcass efficiency and optimal ruminal parameters.