In the first section of this chapter, the function of calcium in the human body, food nanotechnology, and several traditional analyzed methods for nanocalcium were described. The normal methods could determine the concentrations of nanocalcium additives in food and the analytic species is in ionic or ionized state. However, it is not easy to evaluate the sources of the calcium salt. To find the correct structures of added calcium salts, it would be one of the important issues to identify the phase transformation of the calcium during any treatments. X-ray diffraction technology can provide useful structural information of any crystals to detect the quantities of additives in the nanofood or identify the phase variation of food treated by different procedures. Therefore, the basic fundamental principles of the X-ray diffraction technology are shown in the second section. The content of the third section further probes into the crystal structures of nanocalcium additives in milk powders in the recent market. The X-ray diffraction patterns of the milk samples with the addition of nanocalcium have been obtained to evaluate the crystal phase and its structure transformation. In addition, the results also inferred that the milk powders containing nanocalcium after processing would prompt the transformation of crystal structure to partially generate oxide structures or even hydroxide structures. These trends make the mean of structure information of the nanocalcium additives more complicated. Herein, this chapter provides a simple methodology by using the X-ray diffraction technology to carry out the structural identification of the nanocalcium additives in milk powders. It can effectively characterize the phases of components and also reveal the structural stability of the nanoscale additives in food nanotechnology.