An experiment was conducted to examine the effects of adding microbial phytase enzyme (E) [each gram contains 2500 FTU] at 0.05% of corn-soybean meal diets containing varying available phosphorus (AP) levels on the performance of Sinai laying hens. Six isocaloric-(2700 kcalfkg) isonitrogenous—(160/o CP) with 3.3% Ca diets were formulated. Diet 1 contained 0.405% AP (a normal level) and served as a negative control (T1). Diet 2 had the same AP level plus E and designated as a positive control (T2). Whereas, diets 3, 4, 5 and 6 of T3, T4, T5 and T6 contained 0.334, 0.262, 0.190 and 0.119% AP, respectively, plus E. Each diet was fed to six replicates of 5 birds each from 32 to 52 weeks of age. No significant differences were observed among dietary treatments in the performance of Sinai laying hens foe egg production, egg weight, egg mass, feed intake and feed conversion, with the exception of those of T6 which gave a significantly lower egg weight. As for reproductive performance; egg fertility, hatchability, and embryonic mortality were not affected by the dietary treatments. Neither exterior nor interior egg quality traits were influenced by the dietary treatments, as measured at 36, 40 or 44 weeks of age. No significant differences were detected among the dietary treatments in blood constituents of 52-wk-old Sinai laying hens; as assessed by the concentration of plasma total protein, total lipids, cholesterol, Ca and P. in addition, dietary treatments had no effect on carcass traits of these experimental birds. Economically, T2, T4, T5 and T6 exhibited a higher economic efficiency, but T6 had the advantage over all dietary treatments. It would be concluded that Sinai laying hens can be fed on a layer diet containing as low available phosphorous level as 0.119% plus 0.05% Exogenous supplemental phytase without any adverse effect on their productive and reproductive performance.