The goal of this study was to evaluate the effects of variable dietary metabolizable energy and crude protein concentrations on the performance, behaviour, expression of growth related genes and economic impact of broiler chickens. A total of 250 Cobb broiler chicks were divided randomly into five groups of ten replicates (10 chicks/replicate). Five diets with five different combinations of calories and protein were formulated as the following; normal energy and normal protein (NENP), low energy and high protein (LEHP), high energy and low protein (HELP), normal energy and low protein (NELP), low energy and normal protein (LENP) during starter and finisher period. The results revealed that the highest body weight and body weight gain were observed in the HELP and NELP groups which were... similar in improvement of feed utilization and protein efficiency ratio. The total feed intake was increased with decreased dietary energy as in NENP group. Nutrient digestibility was improved with increasing diet energy density as in HELP and NELP groups. The expression of IGF-1 and growth hormone genes were markedly higher in HELP and NELP groups, while the myogenin expression significantly increased in HELP, NELP and LEHP groups. Moreover, the final weight greatly correlated with the gene expression related to growth. Eating and drinking frequencies were highest in LENP and LEHP groups. Birds reared in HELP diet were more active, as expressed by greater walking, wing shaking, leg stretch, preening and flying. Briefly, our results suggested that the energy of diets greatly affected the broiler performance, behaviour and digestibility. Additionally, inclusion of low protein diet with addition of critical amino acids had a positive effect on economic efficiency of broilers. Thus, NELP and HELP diets are recommending for growth of Cobb broiler and could have a significant economic impact.