Abstract: The effects of microbial phytase supplementation on growth performance, ileal digestibility of protein and ether extract, amount of excreted phosphorus and bone development was investigated in a 3 weeks trial using 192 mixed sex, day-old Ross 508 broiler chicks fed corn-soybean meal diet. The trial was set up according to completely randomized design. Day-old chicks were randomly distributed into three story cage units with wire floor. Thus, each of dietary treatment had 8 replications in which 8 birds were assigned. Three dietary treatments according to available phosphorus (AP) levels were respectively, A) positive control (0.45% AP), B) negative control (0.35% AP) and C) negative control+Phytase (0.35% AP) in diets. Phytase enzyme was added at 0.5 g/kg level into related feeds. Phytase supplementation did not significantly (P>0.05) affect bird growth performance Phosphorus and calcium availability were significantly (P<0.05) affected by dietary treatments. Supplemental phytase significantly (P<0.05) improved ileal digestibility of crude fat, but did not have any effect on toe ash content.