The low-temperature (298-673 K) growth and structure of epitaxial delta-NbN thin films (100-800 Angstrom) deposited by reactive sputtering on the (100) NaCl cleavage surface have been studied by transmission electron microscopy. The films have a parallel epitaxial orientation (100)(NaCl)parallel to(100)(NbN) and contain microtwin interlayers in the(111) planes. The microtwin content dependence on the background pressure as well as on deposition rate has been found. It has been concluded that microtwins form at the initial growth stage as a result of coalescence of islands of different orientations, (100) and (111), like condensed fee metal films. The high density of block boundaries and the distortion of coherent growth with small film thicknesses (h approximate to 100 Angstrom) are attributed to the high activation energy of coalescence and recrystallization processes characteristic of refractory compounds.