The E-fields in a loaded GTEM (gigahertz transverse electromagnetic) cell and an anechoic chamber with an equipment under test (EUT) were calculated by using the FDTD (finite-difference time-domain) method at frequencies of up to 1 GHz when the EUT was modeled by a conductive cube. A coefficient is proposed that is intended to evaluate the difference between an average of the E-fields on the six surfaces of the EUT in a case where the EUT is loaded and unloaded. As a result, the maximum deviation from the coefficient in the anechoic chamber for that in the GTEM cell became larger as the EUT size was increased, and the deviation was less then 5.0 dB when the ratio of an EUT size to a distance between the septum (inner conductor of a GTEM cell) and the floor conductor was below 0.4.