The high resolution transmission electron microscope(HRTEM) is one of the most powerful methods fer investigating internal structures of various materials on an atomic scale. In fact, HRTEM images are becoming much more common in scientific papers, and are making valuable contributions to development of industrial products. With rapid improvement of current HRTEMS, their maximum resolution reaches almost 0.1 nm. In this paper we describe the fundamental formulation of the imaging process of HRTEM and their practical application f3r nano materials.