The static, dynamic and attenuation properties Of Co(2)Z barium ferrites and Co(2)Z composites have been studied. The results showed that both static and dynamic magnetic properties are significantly different for large particles and nanoparticles. As compared to large particles, Co(2)Z nanoparticles have a small saturation magnetization M, large coercivity H-c small permeability mu'(0) and mu"(max), but high resonance frequency f(R). The maximum reflection loss predicted is much smaller for nanoparticles than for large particles. Therefore, Co(2)Z barium ferrite with large particle is more suitable for EM materials with high attenuation and broad bandwidth at microwave frequency. (C) 2005 Springer Science + Business Media, Inc.