With the development of nanometer science and technology, some specific characteristics of nanometer substances have been revealed, which, however, are attributed to their crystal structures and crystal chemistry characteristics. To detect specific characteristics of the nano-talc, we must base our study on the crystal structure and crystal chemistry theory, and then give some theoretical foundation to the determination of optimum talc particle dimension. According to the cell parameters and the atomic arrangement of talc, the authors put forward the minimum dimension of talc nano-particle. By calculating the crystal cell number, the atomicity, the surface layer atomicity and its proportion, this paper deals with the relationships between these factors and the dimension of talc. The optimum dimension of talc nano-particle is assumed to be about 165 nm.