Atomically-flat surface (0001) aluminum nitride (AlN) epitaxial films grown by metalorganic chemical vapor deposition (MOCVD) were investigated. To clarify the growth mechanism of the AlN film, dependences between growth rate and process parameters of MOCVD were evaluated. To investigate the relationship between surface roughness and dislocation, three types of dislocation were observed by transmission electron microscopy (TEM). It was found that suppression of the thermal convection was indispensable for growth of atomically-flat surface AlN. From TEM observation, periodical misfit-type dislocation was observed only within the 10-A-thick interface between AlN and sapphire substrate. The density of edge-type dislocation in the AlN surface region was as low as 1010-1011 cm-2. Screw-type dislocation mostly stopped propagating along AlN within the thickness of 0.5 μm which agreed with the thickness of changing rough surface AlN to atomically-flat surface AlN. It is considered that suppression of screw-type dislocation was contributed to atomically-flat surface AlN.