Zn0.92Mn0.08O nanoparticles were synthesized by co-precipitation technique, and annealed in H-2, CO, Ar and O-2, respectively. Room temperature ferromagnetism was found in samples annealed in H-2 and CO, respectively. The absorption spectra and Raman spectra of the samples reveal that a high concentration of oxygen vacancies appears in CO-annealed and H-2-annealed Zn0.92Mn0.08O nanoparticles. By Gaussian fitting PL spectra of the samples, a broader green-yellow emission band decomposed into the green emission band and yellow emission band, providing the evidence that singly ionized oxygen vacancies may play an important role in the origin of room temperature ferromagnetism. The exchange interaction between the donor electron trapped by the singly ionized oxygen vacancy and surrounding Mn ions is responsible for ferromagnetism of diluted magnetic semiconductors at room temperature. (C) 2012 Elsevier B.V. All rights reserved.