Recently, site evaluation of anechoic chamber for EMI test over 1 GHz has attracted much attention because EU and Japan have started the regulation for the upper frequency limit of EMI from 1 GHz to 6 GHz from October 2010. The evaluation method defined by Comité International Special des Perturbations Radioélectriques (CISPR) is not enough for the detailed analysis of the anechoic chambers for EMI test over 1 GHz because it cannot identify the cause of reflections. To solve the problem, we have developed a new site evaluation method using plane-wave spectral decomposition technique and an optical microwave transceiver. The proposed method enables us to obtain the intensity and the angle of arrival of reflection waves and facilitates improving performance of anechoic chambers in the frequency range from 1 GHz to 18 GHz.