In this work, the full energy peak efficiency for a system of two NaI gamma detectors using spherical radioactive sources is evaluated by applying a new numerical algorithm method, since experimental calibration using volumetric sources is a dominant problem of practical gamma spectrometry. The new method is based on the efficiency transfer technique, where the effective solid angles, the effect of self-absorption of the source matrix, and absorption by the source container and the detector housing materials on detector efficiency are considered. The experimental calibration procedure was done using radioactive spherical sources containing aqueous 152Eu radionuclide, which produces photons with a wide range of energies from 121.78 up to 1408.03 keV. The comparison shows good agreement between the measured and calculated efficiencies for the detector using spherical sources.