The nitrogen in polycrystalline diamond films prepared using the direct current arcjet plasma process has been studied using scanning electron microscopy, X-ray diffraction and electron spin resonance (ESR). The ESR spectrum of single substitutional C nitrogen was observed, whose intensity showed that the spin concentration decreases with an increase in the substrate temperature. The preferentially orientated growth is influenced by nitrogen. The result indicates that nitrogen plays a special role in the formation and structure of the polycrystalline diamond films studied in this report.