To calibrate a whole-body counter. it is necessary to find a determination method for peak efficiencies of detectors used in the whole-body counter. For this purpose. peak efficiencies of a Ge semiconductor detector for point and volume Sources were evaluated in the photon energy range 60-1836 keV by Monte Carlo simulation and experiment. It was found that the calculated peak efficiency curves as a function of energy without modelling of the actual sensitive region of the detector are similar in shape to those measured. The calculated peak efficiencies of the detector having an apparent dead layer (I unit) were also found to agree with the experimental values (deviations from -10-24%). Consequently, the simulation method for peak efficiencies was validated. In addition, an optimum design for a whole-body counter with Ge semiconductor detectors was examined by simulation. This simulation provides a method to determine an optimum arrangement of detectors in a whole-body counter, offering a uniform response to various Cs-137 distributions in a human body.