A total of 780 unsexed day old Arbor Acres chicks were used. The birds were randomly distributed into 26 treatments of 30 chicks each. Two studies were designed to compare three phytase sources differed in their microbial origin and thermo-stability on performance and bone mineralization of broilers. In Exp. 1, with less 40% available phosphorus (aP) diets than recommendation, chicks were fed 0.3% and 0.27% aP (-40% aP), during starter and grower periods, respectively, and supplemented with one of the following phytases; Phytase I (Schizosaccharomyces pombe), Phytase II (Trichoderma reesei) and Phytase III (Pichia pastoris) at 3 levels (0, 500 or 750 FTU/kg diet) each within two feed forms (mash or pellet) in 3×3×2 factorial design. In a concurrent study, with less 20 or 40% aP with the most stable phytase source from Exp. 1 at 0, 500 and 750 FTU/kg diet, also in mash or pellet feeds (Exp. 2) in 2×3×2 factorial arrangement. In both experiments, two control groups fed strain recommendation of aP (contained 0.5%, 0.45% aP during starter and grower periods, respectively) with the same feed forms were used for comparison. Results obtained indicate that: Experiment 1: There is a difference in thermo-stability among phytase sources according to the microbial origin. 1-                  Chicks fed -40% aP and supplemented with either Phytase I or Phytase III  had significantly higher weight gain and carcass percentage than Phytase II. 2-                  Chicks fed pellet diets recor ded (P≤0.05) better growth performance compared to those on mash diets. It could be concluded that: phytase I (Schizosaccharomyces pombe ) is the most stable phytase, while, chicks fed -40% aP, irrespective  of phytase source didn't reach the values of control group (strain recommendation of aP) concerning growth performance parameters or bone measurements. Experiment 2: Chicks fed -20% aP diet improved feed conversion ratio (FCR) by 4.1% for overall period and higher plasma P and Ca by 34.8% and 27.7%, respectively comparing to others fed -40% aP. 1-                  Chicks fed dietary Phytase I at levels of 500 and 750 FTU/kg diet improved significantly live body weight, carcass % and better overall FCR. 2-                  By feeding pellet diet, chicks recorded better overall FCR by 4.1% than those fed mash diet. It could be recommended that addition of coated Phytase I (Schizosaccharomyces pombe) at 500 FTU/kg diet to -20% aP diet improved growth, bone mineralization of broiler chicks fed either mash or pellet diets.