Six weeks feeding trial using 210 one-day old unsexed Cobb 500 chicks was carried out to study the effects of addingphytase enzyme on growth performance, blood parametersand bone characteristics in broiler chicks. Three starter diets were used from 1 to 21 days; T100 {100% of Ca & non-phytate phosphorus NPP requirements (1.00% Ca and 0.50% NPP)}; T75{75% of Ca & NPP requirements (0.75% Ca and 0.38% NPP)} and T50 {50% of Ca & NPP requirements (0.51% Ca and 0.25% NPP)}. Three grower diets were used from 22 to 42 days; T100 {100% of Ca & NPP requirements (0.91% Ca and 0.46% NPP)}; T75{75% of Ca & NPP requirements( 0.68% Ca and 0.34% NPP)} and T50{50% of Ca & NPP requirements (0.45% Ca and 0.23% NPP)}. Seven dietary treatments were distributed according to diets fed consecutively during starter and grower phases as; (100/100) fed starter T100 then grower T100; (100/75) fed starter T100 then grower T75; (100/50) fed starter T100 then grower T50; (75/75) fed starter T75 then grower T75; (75/50) fed starter T75 then grower T50 and (50/50) fed starter T50 then grower T50.All diets added phytase enzyme (FTU 10000/Ton). Each treatment comprised of 30 chicks in 3 replicates of 10 chicks each. Results showed that live body weight and weight gain did not significantly affected by restricted Ca and NPP diets at starter period, where weight gain was significantly affected at grower period. Birds group received diet containing 100/75 with phytase (T3) gave the highest significant values for LBW and BWG, then 50/50 with phytase (T7) as compared with the other experimental groups during the grower and overall periods.Feed consumption and conversion ratio values were no significant differences among groups fed different dietary treatments during starter and grower phases. Plasma P concentrations was significantly affected however plasma Ca concentrations, GOT and GPT valueswere not significantly affected by the reduction of dietary Ca and NPPwith adding phytase. Percentages of tibia ash, Ca and P were significantly affected by dietary treatments. Supplementation of phytasehas a significant effect ontibia breaking strength, while supplementation of phytase hasn’t any effect on tibia Seedorindex.It is obvious that, the best performance was seen with (100/75) diet without any adverse effect on productive performance, blood parameters and most of tibia measurements and chemical composition.