With the development of nanoscience and nanotechnology, the study on nano pyrophyllite is of great importance. According to the cell parameters and the atomic arrangement of pyrophyllite, the minimum size of pyrophyllite nanoparticle was supposed. Moreover, the crystal cell number, atomicity, the atomicity on the superficial layer of (001) crystal plane and its proportion for pyrophyllite nanoparticles of different nanosizes are theoretically calculated, and their variations with the nanoparticle size as well as their influence on the overall property of pyrophyllite are analyzed. Based on crystal structure and crystal chemistry theory, the optimum pyrophyllite nanoparticle size was theoretically determined to be about 155 nm.