Abstract: Background and Objective: This study aimed to determine the energy and protein balance in male Simmental cattle when fed diets with different protein and energy levels. Methodology: The experimental design used a randomized block design (RBD) with a 2×3×3 factorial pattern. There were two factors, namely, Factor A, which was the level of dietary energy (65 or 70% TDN) and Factor B, which was the level of dietary protein (10, 12 or 14%). Each treatment was repeated 3 times. The diet consisted of ammoniated rice straw and a concentrate and was fed at a ratio of 40:60. The concentrate consisted of palm kernel cake, rice bran, cassava, mineral and urea. The measured variables were intake; the digestibility of dry matter (DM), organic matter (OM) and crude protein (CP), daily gain, urine allantoin, blood metabolites and feed efficiency. Results: There was no interactive effect between the protein and energy levels of the diet on nutrient intake (DM intake, OM intake and CP intake), weight gain, or efficiency (p<0.05). There was an interactive effect between the dietary protein and energy levels on nutrient digestibility (DM, OM and CP) and urine allantoin (p>0.01). Nutrient intake tended to decrease with increasing levels of energy and protein in the diet, while nutrient digestibility increased with increasing energy levels in the diet but did not significantly change with increasing protein levels in the diet. The average daily gain and feed efficiency improved with the increasing levels of energy in the diet. There was an interactive effect (p<0.05) between the energy and protein levels on the cholesterol and HDL levels. Cholesterol levels increased with increasing levels of protein in the ration. There was no interactive effect (p>0.05) between the protein and energy levels on LDL, total protein, glucose, albumin, globulin, calcium or urea. Conclusion: It can be concluded that treatment A2B2, namely, an energy level of 70% TDN and a protein level of 12%, could provide the best reproductive performance of Simmental cattle in tropical areas.