Chloritoid, collected from the argillite in the bottom of Yaopo Formation of Western Hills of Beijing, was studied based on the polarizing microscope, X-ray diffraction combined with Fourier infrared spectroscopy and electron probe. The results show that chlotitoid is in paragenesis with diaspore, 0.7 nm chlorite, pyrophyllite and illite. Chloritoid under polarized microscope is in the shape of chrysanthemum-like, radial and bundle. Its {001} cleavage and (010) partings are obvious, and the interference colour is the first order grey under the cross-polarizer. The X-ray diffraction pattern indicates this mineral as monoclinic syngony. Its cell parameters are calculated as a=0.948 728 nm，b=0.543 418 nm，c=1.811 236 nm，β=102.019 1°. Its chemical formula can be written as (Fe2+ 1.93Mn0.33Ti0.06)Al3.92Si2.11O12.18 base on the electron probe data. The research shows that:the chloritoid was formed by magmatic thermal metamorphism when granite intruded in the bottom of Yaopo Formation in the late Mesozoic.