This experiment was conducted to compare the effect of diets with various energy constant ME: CP ratio on performance, nutrients efficiency and carcass characteristics of broiler chickens. In a completely randomized design experiment, five starter, grower and finisher diets were formulated to have 2800, 2,900, 3,000, 3,100 and 3,200 kcal of ME/kg whereas the CP in starter diets (0-7 d) were 20.14, 20.84, 21.56, 22.28 and 23%, in grower diets (8-28d) were 17.5, 18.13, 18.75, 19.38 and 20 and in finisher diets (29-42 d) were 15.75, 16.31, 16.85, 17.43 and 18% respectively. Two hundred day old Arbor acres broiler chickens were randomly assigned to 20 groups of 10 birds each. The ME: CP ratio and other nutrients such as Ca, P, and amino acid per CP ratio were fixed over all diets in every period. Feed and water were fed ad libitum. Body weight and feed consumption were measured weekly, and carcass characteristics were evaluated at the termination of experiment. Two birds (1 male and 1 female) from each pen were randomly selected and slaughtered to weigh carcass yield, gizzard, heart, liver, abdominal fat, breast meat, and thigh meat at the end of experiment. Body weight was greater in birds fed diets contained more than 3000 kcal/kg compared to those fed lower than 3000 kcal ME/kg. The feed intake was not affected by dietary treatment during growing and finishing periods. Birds fed diets contained lower energy and protein had a higher FCR during all periods. The energy efficiency ratio (EER) and protein efficiency ratio (PER) were decreased in birds fed diets with low CP and ME content during growing period but not affected throughout starter and and finisher periods. Dietary treatments did not influence relative weight of thigh, breast bile, gizzard, and abdominal fat, but the relative weight of liver and heart increased (P <0.05) when chickens fed diet contained higher ratio of ME: CP. The gender did not affect percentage weight of liver, Gallbladder, gizzard and abdominal fat, but the percentage of breast, thigh and heart were influenced by sex of birds. It is concluded that the high nutrients density and high energy diets may improve the nutrients efficiency, carcass characteristics and performance of broiler chickens.

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