The nanosized Fe magnetic particles were successfully synthesized by traditional carbonyl iron manufacture technique. The active nanoparticles were then effectively passivated to be Fe/Fe3O4 composite structure. The structural characteristics were probed by X-ray diffraction, and transmission electron microscopy. High-resolution images show that the granular composites exhibit a classical nanoscale chained characteristic. The size of iron core is about 10-20 nm with a Fe3O4 superficial layer no more than 5 nm. The room temperature hysteresis measurement indicates that small-size induced magnetic characteristics of these Fe/Fe3O4 composite powders are very attractive for the potential high frequency application. The electromagnetic properties of the powders measured by a vector network analyzer in the centimeter wave frequency range are also reported in detail.