Technical Abstract: Although calcium (Ca) supplementation increases bone density, the increase is small and the impact on bone strength and fracture risk is uncertain. To investigate if bone mass, morphology, and biomechanical properties are affected by deficient to copious dietary Ca concentrations, the long bones (tibias and femurs) of growing female Sprague-Dawley rats (8/group) were assessed after 13 wk consuming 1, 2, 3, 4, 5, 6, or 7 g Ca/kg of a modified AIN-93G diet. Dietary phosphorous (P) and vitamin D remained constant at recommended concentrations. The assessment included mineralization, density, biomechanical properties of breaking by a three-point flexure test, and morphological properties by micro-CT scanning of trabecular bone of the proximal tibia metaphysis. Food intake, weight gain, renal and muscle Ca content, and bone hydroxyproline, were unaffected by dietary treatment. All bone parameters measured were significantly impaired by Ca deficiency when diets contained 1 g Ca/kg. Modest impairments occurred with some parameters (bone density, biomechanical bending moment, modulus of elasticity, and stress) at 2 g Ca/kg, but all parameters stabilized between 2 and 3 g/kg diet, with no differences observed between 3 and 7 g/kg. The results suggest that a threshold response in bone Ca retention or bone mass at approximately 2.5 g Ca/kg diet is associated with similar threshold responses in bone breaking strength and related biomechanics as well as trabecular structural properties. There was no evidence of a relative P deficiency or of improved or impaired bone strength and structure as Ca intakes increased beyond those needed to maximize bone density.