The present experiment was conducted to evaluate the effect of adding different levels of microbial phytase and available phosphorus on some physiological and immunological parameters of broiler chicks. A total of 270 one-day-old chicks was divided into nine treatments of 30 birds each, which subsequently were subdivided Into 3 replicates, 10 birds each. The nine treatments were 1- 0.45 AvP%+ 0 FU (control group), 2-0.45 AvP%+ 500 FU, 3- 0.45 AvP% + 750 FU, 4- 0.30 AvP%+ 0 FU, 5- 0.30 AvP%+ 500 FU, 6.030 AvP% + 750 FU, 7- 0.15 AvP% + 0 FU, 8- 0.15 AvP%+ 500 FU, 9- 0.15 AvP%+ 750 FU. The experiment was run rfor 6 weeks. Body weight, feed consumption and feed conversion were recorded weekly. At the end of experiment (6 weeks), three birds were randomly taken from each treatment and slaughtered. Plasma calcium, phosphorus, cholesterol, total lipids, total protein, acid, alkaline phosphates and some blood constituents (hernatocrit %, hemoglobin %, Red blood cells, (10'm3), M.C.H. (1015L), M.C.V. (102g) andM.C.H.C. % were measured at 6 weeks of age. The main results can be summarized as follows: - The higher level of microbial phytase gave significantly the best live body weight and weight gain at 2 and 6 weeks of age in the group fed 750 FU phytase level, no significant difference between the two phytase levels (500-750 FU) was detected at 4 weeks of age. - The low P diet (0.15%) significantly decreased feed consumption of broilers at 2, 4 & 6 weeks of age, while, the groups fed 500 & 750 FU phytase levels showed high feed consumption and better feed conversion compared with the control group at 0-6 weeks of age. - The birds fed low P (0.15%) and high phytase (750 FU) diet has significant higher relative weight of thymus, however thyroid gland, spleen and bursa relative weights were not affected. - The shank relative weight and tibia length were higher In the group fed (0 FU) phytase and high P diet (0.45 %) compared with other groups fed high phytase and low P diet, while, no significant differences in tibia weight and dry tibia weight were observed in all groups. - At 6 weeks of age, the plasma calcium, total protein, alkaline phosphatase in the group fed 500 FU phytase were