For this study 195 day-old broiler chicks were randomly divided into three treatment groups with five replicates in each treatment group having 13 chicks per replicate per pen. Three iso-nitrogenous and iso-caloric rations were prepared as: R-I served as control with 0.83% di-calcium phosphate (DCP); R-II contained 50% less DCP (0.41%) than R-I and R-III contained R-II plus phytase Quantum® Blue @ 100 g/ton feed. Daily feed intake and weekly body weight gain per replicate were recorded for 35 days. On 35th day, blood and tibia bone samples from 5 birds per treatment were collected. Total weight gain and dry matter intake in chicks fed rations I, II and III were 1861, 1736 and 2023 gm/bird and 3242, 3284 and 3265 gm/bird, respectively. The chicks fed R-III got 16.00 and 8.00% higher weight gain compared to chicks fed R-II and R-I, respectively. Feed: gain ratio (gm/gm) was found best (P<0.05) in chicks fed R-III (1.61) and poorer in chicks on R-II (1.89). The concentration of Ca and P were 10.60, 9.47 and 11.43 mg/dL and 5.76, 4.91 and 7.11 mg/dL in blood serum and 32.61, 31.36 and 32.83% and 16.57, 15.92 and 16.60% in tibia bone of chicks fed rations I, II and III, respectively. The total feed cost per unit weight gain of broilers fed R-III diet was numerically 14% and 9% less than the broilers fed R-II and R-I, respectively. Results suggested that exogenous microbial phytase supplementation to rations having low DCP had positive effects on weight gain, feed: gain ratio and economic efficiency in broiler chicks.