Previous studies have revealed that magnesium (Mg) plays a significant role in bone health; however, few studies have investigated the effects of Mg supplementation in diets with different calcium (Ca) levels on the bone status and bone metabolism in a growing stage. In this present study, we tested the effects of Mg supplementation on bone status in growing female rats, relative to Ca intake levels. A total of 40 Sprague-Dawley female rats aged 6 weeks were divided into the following four groups and fed for 12 weeks as indicated: (1) LCaAMg: low Ca (Ca, 0.1 % of total diet) and adequate Mg (Mg, 0.05 % of total diet), (2) LCaHMg: low Ca and high Mg ( Mg, 0.1 % of total diet), (3) ACaAMg: adequate Ca (Ca, 0.5 % of total diet) and adequate Mg, and (4) ACaHMg: adequate Ca and high Mg. Our results showed that Mg supplementation with the adequate Ca diet significantly increased the bone mineral contents, bone size (bone area and bone thickness), and bone mineral density of femur or tibia by improving bone metabolism without changing Ca absorption. Mg supplementation significantly increased the serum osteocalcin in the adequate-Ca-diet group (p < 0.05), while the Mg supplementation significantly decreased the serum level of C-telopeptide cross-links of type I collagen in the adequate-Ca-diet group (p < 0.001). This study suggests that Mg supplementation with adequate Ca intake in the growing stage may increase the bone mineral density and bone size by improving bone metabolism.