An experiment was carried out to investigate the effect of feeding some different plant diets containing different metabolizable energy (ME) supplemented with an enzyme preparation (Phytase) on growth performance, nutrient digestibility, carcass traits, some bone and blood parameters of broiler chicks. The current study was designed as a 2 x 3 factorial arrangement by using different levels of ME (3000, 2900 and 2800 kcal/kg starter diets and 3100, 3000 and 2900 kcal/kg grower diets and 3200, 3100 and 3000 kcal/kg finisher diets), with two levels of phytase (0 and 750 FTU/kg diet). Diets were formulated to be isonitrogenous, containing 23% and 21% and 19% crude protein during starter, grower and finisher periods, respectively. The obtained data showed that regardless of phytase, decreasing dietary ME level in starter, grower and finisher periods 100 or 200 kcal/kg below control negatively affected economic efficiency and significant reduced percentage of abdominal fat, final live body weight, body weight gain, feed intake, feed conversion ratio, liver percentage, spleen percentage and had no significant effect on all carcass traits and carcass parts or blood parameters of broiler chicks except triglyceride, total lipids, uric acid and GOT and finally length and width of tibia. Phytase supplementation had significant effect on weight gain, tibia ash, tibia Ca, tibia P, plasma P and digestibility of crude fiber, ether extract and ash retention. According to the economical study feeding chicks balanced diets supplemented with microbial phytase at level of 750 FTU/kg is more successful in view of growth and economical evaluation of broiler chicks.