Transparent p-type nickel oxide thin films were grown on polyethylene terephthalate ( PET) and glass substrates by RF magnetron sputtering technique in argon + oxygen atmosphere with different oxygen partial pressures at room temperature. The morphology of the NiO thin films grown on PET and glass substrates was studied by atomic force microscope. The rms surface roughnesses of the films were in the range 0.63-0.65 nm. These ultra smooth nanocrystalline NiO thin films are useful for many applications. High resolution transmission electron microscopic studies revealed that the grains of NiO films on the highly flexible PET substrate were purely crystalline and spherical in shape with diameters 8-10 nm. XRD analysis also supported these results. NiO films grown on the PET substrates were found to have better crystalline quality with fewer defects than those on the glass substrates. The sheet resistances of the NiO films deposited on PET and glass substrates were not much different; having values 5.1 and 5.3 k Omega/square and decreased to 3.05, 3.1 k Omega/square respectively with increasing oxygen partial pressure. The thicknesses of the films on both substrates were similar to 700 nm. It was also noted that further increase in oxygen partial pressure caused increase in resistivity due to formation of defects in NiO. (C) 2009 Elsevier B.V. All rights reserved.