We prepared a novel type of milk calcium from milk whey, and evaluated its calcium bioavailability as compared with conventional industrial milk calcium obtained from milk whey, which has been reported to be an excellent calcium source. An X-ray diffraction analysis showed that the chemical form of conventional industrial milk calcium from milk whey through the neutralization and precipitation method (NP-MCa) is hydroxyapatite type, and that of a novel type of milk calcium obtained using the ultrafiltration method (UF-MCa) is different from hydroxyapatite. Growing rats were fed either NP-MCa or OF-MCa as the source of calcium for 33d. The calcium absorption rate in the OF-MCa diet group was significantly higher than that in the NP-MCa diet group. The calcium content of the femur in the OF-MCa diet group was significantly higher than that in the NP-MCa diet group. The breaking force of the excised femur in the OF-MCa diet group was also significantly higher than that in the NP-MCa diet group. The serum calcium level, alkaline phosphatase activity and calcitonin level were not significantly different between the experimental groups, but the PTH level of the serum in the OF-MCa diet group was significantly lower than that in the NP-MCa diet group. These results indicate that the calcium bioavailability of the novel type of milk calcium (UF-MCa) is higher than that of the hydroxyapatite type of milk calcium (NP-MCa).