This experiment was conducted to examine the effects of 1α-OH-D₃ either with or without cholecalciferol as well as two levels of dietary calcium (Ca) and phosphorus (P) on broiler chickens performance and tibia parameters. A total of 192 one day old mixed sex broiler chicks (Ross 308) were allocated to four treatments, each with 4 replicates. The dietary treatments were as follow: T₁: Ca-P-adequate + phytase; T₂: Ca-P-adequate + phytase + 1α-OHD₃; T₃: Ca-P-deficient + phytase + 1α-OHD₃; T₄: Ca-P-deficient diet without cholecalciferol + phytase + 1α-OHD₃. The feed conversion ratio (FCR) was significantly improved in starter period and decreased in grower period in broiler fed Ca-P deficient diet without vitamin D₃ when compared with the same diet with vitamin D_3. The broilers fed the deficient diet with vitamin D₃ throughout the starter period had lower body weight and FCR and ate more compared with broilers fed the adequate diet plus1α-OHD₃ (P>0.05). Chicks fed the deficient diet with vitamin D₃ were unable to achieve the same tibia ash, Ca and P values as chicks fed the deficient diet without vitamin D₃ (P<0.01). These values for chicks fed the deficient diet without vitamin D₃ were not significantly different from chicks fed adequate diets. In conclusion, the results indicate that Ca-and P-deficient diet without vitamin D₃ supplemented with 5 μg/kg of 1α-OH D₃ and 500 FTU/kg of phytase appear to be adequate for broilers' bone mineralization, however, there are some variables that may limit growth performance of broilers.