Abstract: Calcium is one of the essential elements that form human bone (the main form of calcium is Ca₁₀(PO₄)₆(OH)₂)), and it participates and regulates many life processes. Osteoporosis is the most common disease of calcium deficiency, while recent research shows that organisms(especially osteoporosis organisms) take in too much calcium, possibly causing some other diseases. Thus, the accuracy of calcium absorption rate measurement is very meaningful for reasonable calcium supplement and prevention diseases of calcium metabolism. Based on the high sensitivity of ⁴¹Ca measurement with accelerator mass spectrometry and the innovative methods of ⁴¹Ca labeling endogenous calcium, the calcium absorption rate of rats was studied. The results show that rats intake 28 mg calcium carbonate, 28 mg calcium citrate and 10 mg calcium citrate per 100 g body weight each day, the apparent absorption rates are (67.57±2.79)%, (65.21±2.96)% and (87.50±1.10)%, the static absorption rates are (73.19±3.60)%, (71.47±4.01)% and (92.74±1.81)%, respectively. This method can obtain the true absorption of calcium without extrinsic labeling, and it is not affected by the chemical structure and sorts of calcium supplements.