Spherical iron particles of three different size distributions, 6–10 μ in diameter, 100 mesh and 30–80 mesh, were mixed with 2.0 wt % of soluble imide and compression molded at 300 °C under 131 MPa. Post-fabrication heat treatments were performed at 960 °C for 6 h resulting in a significant enhancement of the permeability in low field region for all the specimens except for the one made of 30–80 mesh particles. The rate of core loss of these specimens at a magnetic induction of 5 kG measured up to 1 kHz shows a noticeable increase after heat treatment which, along with the permeability enhancement, can be explained by the coalescence of particles forming a network of conductivity paths in the specimens. The scanning electron micrographs taken for the 6–10 μ particle specimens show no evidence of heat treatment-induced grain growth. The untreated specimens show a very weak f2-dependence of the core loss which clearly indicates a negligible contribution from the eddy current loss. In particular, an almost perfect linearity was found in the frequency dependence of the core loss of the untreated specimen made of 100 mesh iron particles. © 1998 American Institute of Physics.