A lamellar structural unit model of PST crystal used in finite element analysis has been built on the basis of studying the microstructure of PST crystal. The calculation indicates that as crack is parallel to the lamellae, the resolved shear stresses on all slip planes ahead of crack tip are low, and the slip is difficulty, while the normal stress on lamellar interface is higher than that on other cleavage planes, which results in crack nucleation along lamellar interfaces preferentially and low ductility. As the crack perpendicular to the lamellae, the resolved stress on many slip planes ahead of crack tip is much higher, and the slip is easy. The maximum normal stress on the translamellar cleavage planes rather than the lamellar interface, which results in crack nucleation along translamellar interfaces preferentially and good ductility.