The crystalline components of human dental calculus were investigated using microbeam x-ray diffraction analysis. Hydroxyapatite and octacalcium phosphate were most frequently found in that portion having porous and zonal structure. In the portion of the homogeneous illustration showing high calcification, whitlockite is a main component. Brushite was unexpectedly rare, and no calcite was detected in any portion of human dental calculus. The mechanism of the formation of dental calculus being considered in this paper is that octacalcium phosphate or brushite is formed during the initial stage of calcification of dental plaque, and is gradually hydrolyzed and transformed into hydroxyapatite and/or whitlockite.