In this study, the effects of a selected coupling agent on the electromagnetic and mechanical properties of an epoxy composite containing a Ni-Zn ferrite were investigated in terms of content of the coupling agent. The complex permeability, permittivity and electromagnetic attenuation value of the composite were obtained using an impedace/gain phase analyzer(HP4194A) and a network analyzer(HP8753C) in the frequency ranges from 1 MHz to 5 GHz. At a given frequency range, complex permeability, permittivity, and microwave absorbing attenuation of Ni-Zn ferrite/epoxy composite was constant with varying coupling agent amount. On the other hand, tensile strength and hardness value of the composite were greatly affected by the amount of the coupling agent ; the coupling agent treatment up to 1 ∼ 1.5 wt% improved the mechanical properties, however, the effect was saturated above this value. Therefore, without any detrimental effects of coupling agent on the electromagnetic properties at interested frequency range(1 ∼2 GHz), the mechanical properties could be optimized by addition of 1∼1.5 wt% of the coupling agent.