A whole-body counter (WBC) is a device employed in nuclear power plants (NPPs) to identify radionuclides and measure the content of radioactivity in humans. In this study, several experiments were conducted to suggest an optimal and practical method to improve the accuracy of in vivo measurements using WBCs at NPPs. First, countings from the front and back using a phantom were carried out to set up a discrimination programme between internal and external radioactive contamination in NPPs. Second, experiments were performed to select the optimal geometry of the WBC and to locate the contaminated area of radionuclides.