We have prepared ZnO nanofibers by using the electrospinning method and have analyzed the variations in their structure, optical properties, and chemical bonding states with the annealing temperature and the annealing conditions. After annealing at 500 degrees C, the average diameter and the average grain size are determined to be approximately 170 nm and 50 nm, respectively. The ZnO nanofibers have a clearly visible dislocation-free crystal structure. The 2p level of Zn caused Zn-O bonds and the O 1s core level caused physisorbed O-2. When the nanofibers are annealed at 700 degrees C, both ions exhibit shifts of the binding energies compared with those annealed at 500 degrees C. The observed band in the visible region shows a strong temperature dependence and a red-shift with the oxygen flow in annealing process, which is related to oxygen vacancies.