This is the review article on our studies for the external surfaces and internal structure of monodispersed hematite (α-Fe₂O₃) particles by transmission electron microscopy (TEM) on their thin sections prepared with an ultramicrotome. The electron micrographs and electron diffraction patterns obtained from the thin sections of monodispersed peanut-type and pseudocubic particles not only directly revealed their polycrystallinity but also disclosed the shape, size distribution and orientation of the subcrystals in detail. In addition, in the platelet-type single crystalline particles, their basal and side planes could be identified through TEM on their thin sections. Computer simulation of the growth process for the peanut-type particles based on the fundamental growth analysis and the external surface and internal structure of these particles have led us to more detailed and systematic understanding of the growth mechanism. It was also pointed out that the method developed for the structure analysis of the hematite particles could be applied to other monodisperse particles such as Cds particles.