Ultrastructural study of the development of the atrioventricular (AV) node was studied by electron microscopy in human fetus ranging from 30 mm to 260 mm crown rump length, and compared with human adult. By 30 mm fetus, the right AV nodal primordium was located below the attachment of the right venous valve. The left AV nodal primordium was observed below the attachment of septum primum. The cytoplasm of the nodal primordia contained few mitochondria, and myofibrils. These cells were apposed to each other with occasional desmosomes. In 40 mm fetus, the AV node cells were poorly organized myofibrils, while working myocardial cells were well organized myofibrils with sarcomere. At 70 mm fetus, intercalated discs were developed in the working myocardial cells. At 100 mm fetus, the nodal cells contained a relatively clear cytoplasm with a few groups of myofibrils and mitochondria. By $140sim200$ mm fetuses, the nodal cells were an increasing number of myofibrils and mitochondria and these were scattered throughout the cytoplasm. At 260 mm fetus, the nodal cells were small and contained a clear cytoplasm with sparse and poorly organized myofibrils and mitochondria. All major ultrastructural features which characterize the adult AV nodal cells were found in this stage. The working myocardial cells were larger and had a more compact cytoarchitecture than nodal cells. Zonula adherens or fasciae adherens type junction were not found between nodal cells, but they frequently observed between nodal and working myocardial cells.