Gold was deposited in a vacuum on {100} surfaces of magnesium oxide inside a high-resolution electron microscope. The atomic processes of epitaxial growth was in situ observed cross-sectionally for interfaces by time-resolved high-resolution transmission electron microscopy. Growth phenomena of the gold clusters, such as nucleation, structural fluctuation and repeated process of truncation and construction of corners, secondary nucleation and coalescence, were analyzed in real-space at a spatial resolution of 0.2 nm and a time resolution of 1/60 s. Details of the structural fluctuation and related cluster dynamics, i.e., fragmentation, were also demonstrated in this review.