Recent electronic devices and telecommunication system provided some new and serious problems such as EMI, and recent microwave-absorbing materials must be developed to solve these problems. In this study, the effects of volume fraction of ferrite, and particle size on the electromagnetic absorbing behaviors of an epoxy composite containing a Ni-Zn ferrite were investigated. The composites were prepared by the cement mixed method and shaped coaxial, toroidal, and disc type. The complex permeability and permittivity were obtained using an Impedance/Gain Phase Analyzer(HP4194A) and a Network Analyzer (HP8753C) in the frequency ranges from 1 MHz to 5 GHz. Microwave-absorber with attenuation over 10 dB at mobile communication band of 1.9 GHz was designed. The microwave-absorbing behavior of the composite showed that the attenuation curve was shifted to a lower frequency band and the corresponding maximum attenuation value was increased with increasing the amount of the ferrite. The particle size of the ferrite had minor effect on the micorwave-absorbing behavior of the composite. In addition the simulation result of microwave attenuation indicated that it is possible to pre-estimate the microwave absorbing behavior of the composite if the initial permeability and the resonance frequency of composite are known.