Counting efficiencies of a whole-body counter installed in the Japan Atomic Energy Research Institute were evaluated using Monte Carlo simulation and measurement to validate the calculation method of the counting efficiency curves of the whole-body counter for the mater-filled block-shape phantoms containing radionuclides. Consequently, the counting efficiency curves in the photon energy range of 300-1,461 keV mere obtained using the Monte Carlo simulation. The calculations are in fairly good agreement with the measurements in the photon energy range of 662 1,461keV. It was also found that the self-scattering effect of photons in the phantoms accounts fur about 7% of the counting efficiency for 662keV photons.