Effects of feeding modified cassava chip and corn seed as energy source inclusion in diet were investigated on growth performance, rumen fermentation, and blood metabolites in early fattening bulls. Thirty-six 1-year-old Charolais crossbred bulls with initial weight 270 ± 50 kg were randomly assigned into 6 groups with different experimental rations as cassava-based concentrates including non-modified cassava chip (Cass-Con), with 15 % of alkaline-treated cassava chip (Cass-Alkaline), with 15 % of steam-treated cassava chip (Cass-Steam) and corn-based concentrates including non-modified corn seed (Corn-Con), with 15 % of alkaline-treated corn seed (Corn-Alkaline), and with 15 % of steam-treated corn seed (Corn-Steam), according to completely randomized design (CRD). Results showed that feed intake and digestibility were not significantly different among treatments, while digestible dry matter and organic matter intake of Cass-Steam and Corn-Alkaline were higher than those of the other diets (P < 0.05). Ruminal pH post-feeding was highest in Corn-Alkaline and lowest in Cass-Con (P < 0.05). Blood glucose was similar among treatments. However, blood insulin at 4 h post-feeding was higher in Cass-Steam, Corn-Alkaline, and Corn-Steam than in the others. Blood insulin in bulls fed corn-based concentrate was higher than in bulls fed cassava-based concentrate (P < 0.01). Body weight gain and average daily gain were significantly higher (P < 0.05), while feed conversion ratio was lower in Cass-Steam, Corn-Alkaline, and Corn-Steam as compared with in Cass-Con, Cass-Alkaline and Corn-Con. Results indicated that using a modified energy source can improve growth performance in early fattening beef bulls. An appropriate method to modify cassava chip was steam method, while alkaline method for corn seed was superior.