The coalescence of diamond grains grown by microwave plasma chemical vapor deposition was observed at an atomic level using high resolution electron microscopy in combination with scanning electron microscopy. It was shown that large diamond crystals can be grown either by a coalescence of two [001]‐oriented grains which have a slight misorientation or by a switch of the grain boundary, which is usually perpendicular to the surface plane, to a parallel direction. A single crystalline film might be deposited when the growing surface is fully covered by the (001) plane, i.e., no {111} top facet appears to separate the growing (001) surface. ©1996 American Institute of Physics.