ABSTRACT: The present study was aimed to evaluate the calcium bioavailability of pearl powder for humans. Both the nanonized pearl powder (NPP) and the micronized pearl powder (MPP) prepared by a dry grinder were tested. A group of healthy adults free from hyperthyroidism, hypercalcemia, and hypocalcemia were recruited as the subjects for oral administration with the pearl powder. The bioavailability was evaluated by the serum total calcium increment, the serum intact parathyroid hormone (iPTH) reduction, and the urine calcium/creatinine ratio increment in 6 h after administration. The results show better absorption and retention of calcium from NPP, as reflected with the shorter time elapsed before the maximum concentration of calcium appeared in the serum, higher iPTH reduction, more calcium absorption, and higher maximum calcium concentration (Cmax) in serum after ingestion, than that from MPP. We conclude that pearl powder is a beneficial source of calcium for adults and that nanonization improves its calcium bioavailability.