This paper reports the characterization of nanocrystalline ZnO powders synthesized by it precipitation method and annealed with carbon black. The X-ray diffraction (XRD) and Fourier Transformed Infrared (FT-IR) results revealed that the synthesized ZnO powder has the wurtzite structure with absorbed CO3- species on the surface of the ZnO particles. Singly ionized oxygen vacancy (V-o(+)) and CO3- species were also perceived from electron paramagnetic resonance (EPR) analysis. The intensity of the EPR signals of CO3- species increased as the amount of carbon increased whereas that of V-o(+) did not vary significantly. A green emission at 528 nm for the powders annealed with carbon was observed and a good correlation between the intensity of green emission and the intensity of EPR signals of PCO3- was obtained Experimental results suggest that the formation of the free carriers has significant effect on the intensity of the green emission. The mechanism responsible for the green emission enhancement based on the relevance of the observations is discussed. (c) 2007 Elsevier Ltd. All rights reserved.