To test phytase efficiency, 4 dietary treatments including a positive control (T1), negative control (T2, containing 0.10% less total phosphorus than T1), negative control plus 500 FTU phytase (T3), and low-negative control plus 500 FTU phytase (T4, containing 0.13% less total phosphorus than T1) were used. Reducing the available phosphorus level of the broiler diet from 0.47% to 0.37% during the starter, and from 0.37% to 0.26% during the grower-finisher period significantly (P<0.01) depressed growth performance as compared with T1. Phytase supplementation significantly (P<0.01) improved the growth performance of the birds for both starter and grower periods. Tibia ash and phosphorus content in both the T3 and T4 groups were similar to T1 and higher (P<0.01) than in T2. Phytase supplementation of both negative control diets significantly (P<0.01) reduced the phosphorus level in excreta. It can be concluded that with phytase supplementation, the total phosphorus level in broiler diets can be decreased by 0.13%.