The purpose of this study is to separate the hysteresis loss of an iron powder core into contributions from possible loss mechanisms quantitatively. Insulation coated pure iron powders were compacted and annealed to prepare ring-shaped powder cores. Their bulk and crystal grain boundary densities were controlled with the powder and core preparation conditions. The hysteresis loss decreased with a decrease in the grain boundary density, however, a grain boundary pinning model showed that its contribution to the total hysteresis loss was 50% or more, even at the lowest grain boundary density within the range of the experimental condition in this study. The bulk density contribution in this case was evaluated to be less than 10%. The loss mechanisms will be discussed in detail in comparison with steel sheet cores with the chemical composition equivalent to the iron powders.