A designing method of an electromagnetic wave absorber exhibiting over 20 dB attenuation at 1.9 GHz was investigated. Based on the composite compositions attained from the experimental works, the possible ways in which improving the attenuation ability were simulated. That is, in a fixed complex permeability the effects of the real and imaginary part of the complex permittivity on the attenuation ability were studied. In regarding the dimensional and property variances which could be occurred during the fabrication process, their allowances were also given in the design condition.