The measurement of the energy spectrum of cosmic rays is an important tool to reveal the nature of cosmic rays. In order to measure the cosmic rays up to an energy of 10**18 eV, the KASCADE experiment was extended to KASCADE-Grande. It is a ground based detector to measure extensive air showers, which are initiated by the primary particles of the cosmic radiation. In order to obtain the energy spectrum, data from
December 2003 to September 2007 are analyzed by using the method of constant intensity. This method is applied to the number of charged particles in an air shower and leads to a zenith angle independent number of charged particles, which is a measure of the shower size. Simulations of air showers are then used to calibrate the shower size to the primary energy. Thus, the primary energy spectrum in the energy range of 10**16 eV to 10**18 eV is determined. In the energy region from 10**16 eV to 10**17 eV the resulting spectrum and the spectrum measured with KASCADE overlap. Both measurements are in good agreement in this energy range. Due to the extended energy range, the energy spectrum of KASCADE-Grande connects the KASCADE spectrum with the measurements of experiments like AGASA, Auger and HiRes, which measure up to the highest energies of cosmic rays.