The objective of this study was to investigate the effects of increasing doses of a hybrid bacterial 6-phytase on apparent total tract digestibility (CATTD) of energy and nutrients, plasma level of free myoinositol, retention of calcium (Ca) and phosphorus (P), and growth performance of growing pigs fed Ca and P deficient diets. A total of 15 barrows [21.5 ± 1.4 kg BW; Duroc × (Landrace × Yorkshire)] were allotted to a triplicated 5 × 5 Latin square design with 5 dietary treatments and 5 periods per each square. Pigs were individually housed in a metabolism crate. Five dietary treatments consisted of 5 levels of phytase (0, 500, 1000, 1500, and 2000 FTU/kg) supplemented to a basal diet containing nutrients meeting the requirement except for Ca and P. The basal diet was deficient in total Ca and P by 2.2 and 1.7 g/kg below the recommendations from NRC, respectively. Data were analyzed using orthogonal polynomial contrasts using Proc MIXED of SAS and the optimal phytase levels were determined by a broken line analysis using Proc NLMIXED. Increasing phytase supplementation increased (linear: P < 0.01; quadratic: P < 0.01) CATTD of Ca and P and retention of Ca and P. Increasing phytase supplementation increased (linear: P < 0.05; quadratic: P < 0.05) myoinositol contents in plasma. Based on broken line analyses, CATTD of Ca and retention of Ca were increased until phytase supplementation were increased from 0 to 668 ± 65 FTU/kg (P < 0.01) and 0 to 668 ± 69 FTU/kg (P < 0.01), respectively. The CATTD of P and retention of P were increased until phytase supplementation contents were increased from 0 to 780 ± 86 FTU/kg (P < 0.01) and 0 to 773 ± 93 FTU/kg (P < 0.01), respectively. Myoinositol contents in plasma were increased (11.4 to 22.8 µmol/L) until phytase supplementation were increased from 0 to 705 ± 212 FTU/kg (P < 0.05). Increasing phytase supplementation linearly increased (P < 0.05) average daily gain and tended to linearly increase (P = 0.053) gain to feed ratio. In conclusion, supplementation of a hybrid bacterial 6-phytase improved CATTD of Ca and P, retention of Ca and P, complete hydrolysis of phytate, and growth performance of pigs compared with pigs fed a Ca and P deficient basal diet. Supplementation of a hybrid bacterial 6-phytase at a range of 650 to 800 FTU/kg feeds provided the optimal improvement of Ca, P, and phytate utilization.