Baddeleyite niobium oxynitride (NbON) is a possible low-cost alternative to baddeleyite TaON, which is a well-known oxynitride semiconductor with high photocatalytic activity for visible light. In this study, baddeleyite NbON thin films were epitaxially grown on yittria-stabilized zirconia (YSZ) (100) substrates by using nitrogen-plasma-assisted pulsed laser deposition. Low temperature growth at 500 degrees C on a thin self-seed layer, which was deposited at 600 degrees C, was key to obtain baddeleyite NbON with almost stoichiometric chemical composition. X-ray diffraction measurement revealed multiple domain structure of the film with an epitaxial relationship of (100)(NbON)//(100)(YSZ) and [001](NbON)//[001](YSZ). Band gap of the baddeleyite NbON films was 2.3 eV, which is smaller than that of baddeleyite TaON (2.5 eV). The NbON films showed n-type conduction with rather low mobility of 1.6 x 10(-3) cm(2) V-1 s(-1), which is probably attributable to high defect density and grain boundaries.