The present study is concerned with the measurement of the spectroscopic properties of the CdZnTe detectors in a particular energy range since its sensitivity depends mainly on two parameters: energy resolution and detection efficiency. Thus, the energy resolution (in terms of FWHM) and the peak shape (in terms of peak-to-valley (P/V) ratio) were determined from the gamma-ray spectra measured with a 5×5×5 mm3 CdZnTe detector using radioactive disk sources. The detection efficiency of a CdZnTe detector for a disk source geometry was measured in the energy range of 30-670 keV covering almost most of the X-ray and gamma-ray emissions from the radioisotopes medically used in imaging, the enriched uranium and plutonium isotopes, and some activation/fission products in the spent fuel. A semi-empirical function was proposed to describe the energy dependence detection efficiency of CdZnTe detector for the disk source geometry at a given distance. However, the remarkable decreases in the measured efficiency curve for the detector are found especially in higher energies, above 200 keV up to 670 keV. This indicates that there are the losses in the peak areas associated with incomplete charge collection due to mainly crystal imperfections which can be attributed to the drawbacks existed in the conventional CdZnTe detectors.