This study aims to evaluate the effects of cholecalciferol (vitamin D₃, 0, 5, 10, 25, 50, 100, 250, 500, and 1000 mg · kg^–1) and 1α-hydroxycholecalciferol (1α-OH D₃, 0, 2.5, and 5 mg · kg^–1) on growth performance and tibia quality and to compare the relative potency of 1α-OH D₃ to vitamin D₃ in 1- to 21-day-old female broilers fed a calcium (Ca)- and phosphorus (P)-deficient diet. The basal diet contained 0.50% Ca, 0.25% non-phytate phosphorus (NPP) and was not supplemented with vitamin D₃, whereas the control diet contained 1.00% Ca, 0.45% NPP, and 25 mg · kg^–1 vitamin D₃. Dietary vitamin D₃ levels significantly affected body weight gain (BWG), feed intake (FI), serum Ca and tibia parameters. The addition of 5 mg · kg^–1 1α-OH D₃ resulted in greater BWG, FI, and serum Ca as well as tibia ash weight and content, breaking strength, and Ca and P contents, compared with the birds fed the basal diet. Using BWG, serum Ca, tibia ash weight and content, and tibia Ca and P content as the criteria, the vitamin D₃ requirements of 1- to 21-day-old broilers fed Ca- and P-deficient diets were 64.0, 16.2, 173.0, 65.1, 33.1 and 30.0 mg · kg^–1, and the relative potencies of 1α-OH D₃ to vitamin D₃ were 5.03, 2.19, 18.00, 5.14, 4.09 and 3.33, respectively. These data indicate that high levels of vitamin D₃ can spare the use of P in broiler diets and that the potency of 1α-OH D₃ is higher than that of vitamin D₃.