We report here investigations of crystal and electronic structure of as-synthesized and annealed ZnO nanobelts by an in-situ high-resolution transmission electron microscope equipped with a scanning tunneling microscopy probe. The in-situ band gap measurements of individual ZnO nanobelts were carried out in scanning tunneling spectroscopy mode using the differential conductance dI/dV–V data. The band gap value of the as-synthesized ZnO nanobelts was calculated to be ∼2.98 eV, while this property for the annealed nanobelts (∼3.21 eV) was close to the band gap value for bulk ZnO materials (∼3.37 eV). The difference in the band gap value of the as-synthesized ZnO nanobelts and annealed ones was attributed to the planar defects (e.g. stacking faults and twins). These defects can alter the electronic structure by producing localized resonant states that result in band gap reduction.