Fe-based nanocrystalline cores are widely used in pulse transformers, magnetic switches, inductors, and filters in pulsed power. A test stand was developed to measure the energy losses and pulse permeability of toroidal Fe-based nanocrystalline cores. In this paper Fe-based nanocrystalline cores with high and low residual flux were tested and compared. The initial magnetizing curves and maximum hysteresis loops were obtained under a relatively constant magnetization rate of the order of 1-8 T/μs. The loss density and unsaturated permeability are calculated based on the experimental results. The loss density and permeability are fitted and compared with the published data in a similar parameter space. The presented results of Fe-based nanocrystalline cores can be aided in the practical magnetic designs of magnetic switches, pulse transformers, and inductors, but not restricted to those.