This paper is devoted to the X-ray diffraction study of bone fragments of Permian parareptile Deltavjatia vjatkensis obtained from the Kotelnich vertebrate fossil site, one of the richest of the Permian period, which is characterized by the excellent preservation of fossil remains because of their burial in a silty anaerobic environment similar to modern bogs. The bone is well-preserved and consists of fluorapatite, calcite, quartz, and dolomite. The refined apatite unit-cell parameters of a and b-axis (9.3526 ± 0.0001 and 9.3587 ± 0.0001) Å and c-axis (6.8930 ± 0.0001 and 6.8968 ± 0.0001) Å correspond to F-apatite. Crystallinity index determined as the full width at half maximum of the 002 reflection in degrees 2θ is 0.266–0.250, which is typical for Mesozoic vertebrate bones. Apatite crystallite size (length 70.3–74.9 nm, width 30.7–30.3 nm) in fossil pareiasaur bone is larger than in subfossil and recent mammal bone and is in a good agreement with the values for seismosaurus bone. Both crystallite size and aspect ratio (2.3–2.5) are independent of the fossil pareiasaur bone length.