Abstract: In order to solve the problem that the shielding effectiveness testing is difficult in a small cavity, the test error caused by antenna efficiency in nested frequency-stirred reverberation chamber (RC) method was corrected, and the response of a small monopole mounted on the inside wall was deduced with induction electromotive force method, getting a conclusion that a wall-mounted monopole could be used to measure the power in RC. Based on the conclusion and the improved test method, the SE of an electrical module enclosure was measured. The measured <i<S</i<<sub<21</sub< parameters explain that frequency stirring is effective on changing the electromagnetic field, and the mean value of <i<S</i<<sub<21</sub< indicates that energy is uneven and oscillating with frequency change in the low frequency band. The results show that the SE value gradually weakened from 20 dB to 5 dB when the cavity is in over mode with the frequency rising from 3 GHz to 13 GHz. The results are also independent of the antenna location. However, if the antenna efficiency is ignored, the test error can be up to 10 dB. Thus, more attention must be paid in test.