ZnO nanowires were synthesized at low temperature using a thermal chemical vapor deposition technique. The surface morphology of the samples was obtained by scanning electron microscopy (SEM). The nanowire structure was then characterized by x-ray diffraction (XRD) spectroscopy. The as-grown samples were then annealed in argon, oxygen, and vacuum ambient. The samples were examined by Raman scattering and photoluminescence (PL) spectroscopy. Photoluminescence spectra were collected for each annealed sample by using an ultraviolet (UV) laser as excitation and using both UV and visible blue lasers as excitation. A redshift of a near-band-edge emission peak and a thermal quenching of a green band emission were observed. Comparison between the PL spectra of samples annealed under different ambient conditions reveals the possible origins of the green band emission observed in the samples. The effect of annealing on the deep level trapping center is proposed based on the difference between the PL spectra of the annealed and as-grown samples.