Nanobelts are new materials that have a rectangular cross-section and are characterized by widths and width-to-thickness aspect ratios. In this paper, the thickness and aspect ratios of ZnO nanobelts are measured by a conjunction application of convergent beam electron diffraction (CBED) and electron energy-loss spectroscopy (EELS). The thicknesses of thicker nanobelts are first determined by CBED under two-beam diffracting condition, then they are used to determine the electron inelastic mean-free-path (MFP) length, which is 161+/-15nm for ZnO at 200kV. The thicknesses of the thinner nanobelts are then determined by EELS using the calibrated MFP. The results show that the aspect ratio depends on conditions under which the sample was synthesized.