Enclosures of electronic equipment are an important item to suppress the unwanted emissions from digital processing circuits. However, the shielding effect at high frequencies is decreased by slits and apertures of the enclosures. The method for evaluating shielding effect of enclosures above 1GHz was investigated in this paper. A disc-cone antenna operated by an O/E converter was developed and the radiation property was calculated by the method of moment. The results indicated that the deviation between the calculation and the measurement value was within 3dB in the case of that the antenna is operated in the frequency range from 1GHz to 10GHz. The antenna was placed in the enclosure model constructed with wire meshes and the shielding effects were calculated and measured from 1GHz to 3GHz. The results showed that the tendency of the calculated value closely agreed with that of the measured value. Using the antenna, the shielding effect of the PC cases were investigated from 1GHz to 8GHz. The results indicated that the shielding effect is decreased in proportion with the increase of frequency, and the careful design was important to maintain the shielding effect in this frequency range.