Nowadays, the characterization of many materials using transmission electron microscopy at low voltage is very attractive. In this work, we present set of images with atomic-level resolution of the in-situ coalescence phenomenon of gold nanoparticles using a spherical aberration-corrected scanning transmission electron microscopy (STEM) at 80 kV acceleration voltage. High-angle annular dark-field (HAADF)-STEM images with an atomic resolution was successfully obtained. HAADF-STEM images show the coalescence evolution of the gold nanoparticles, where some interesting image contrast characteristics were observed. The main mechanism of the coalescence phenomenon is the surface diffusion, where the twin boundaries of the nanoparticles act as barrier to the deformation. Molecular dynamics study of the coalescence of the Au nanoparticles at low temperature has been carried out. Molecular dynamical simulation of the coalescence phenomenon predicts rotations and surface diffusion of the nanoparticles. Simulated HAADF-STEM images of the coalescence evolution were obtained. Good agreement between simulation and experimental images was found.