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In this paper, the ultrafine spherical FeSiAl soft magnetic composites were prepared by the particles coated with organosilane modified water-soluble silicone resin. The magnetic response of the sample fabricated with 15.36 μm particle is up to 358 MHz, and the real part of permeability exceeds 57. The P cv of this composite is just 604.85 kW/m3 and 606.44 kW/m3 at 200 kHz,100 mT and 1 MHz, 30 mT, respectively. In addition, the DC bias performance improved reaches to 61.3%, comparing to the sample fabricated by the large particles. The domain wall movement dominates the permeability when large particles are employed. However, the spin rotation plays a more important role in the case of small particles. Two stages with different loss mechanisms were investigated for different particle sizes. When the frequency exceeds 70 kHz, the declination of the insulating coating resistivity results in a loss jump, due to the space charge polarization. The FeSiAl crystalline termination at the grain boundary exhibits a symmetry break, thus resulting in a sharp local rise in the anisotropy field which provides a barrier to the domain wall motion and enhance the DC bias properties. This ultrafine FeSiAl composite shows great potential in the MHz magnetic power application. [Display omitted] • Spherical-FeSiAl soft magnetic composites were prepared. The magnetic response of the sample fabricated with 15.36 μm particle is up to 358 MHz. The P cv of this composite is just 604.85 kW/m3 and 606.44 kW/m3 at 200 kHz, 100 mT and 1 MHz, 30 mT, respectively. • It is found that the domain wall movement dominates the permeability when large particles are employed. However, the spin rotation plays a more important role in the case of small particles. • It also clarified a novel phenomenon that a sudden jump in loss curve at the frequency of near 70 kHz is observed for the short circuit of the insulating coating, due to the space charge polarization. • The electrochemical analysis demonstrates that this organosilane modified water-soluble silicone resin hybrid insulating layer can effective reduce the eddy current loss in high frequency. In this work, organosilane modified water-soluble silicone resin coated spherical-FeSiAl soft magnetic composites with different particle sizes were prepared, and the effects of the particle size on the permeability, magnetic loss, and DC bias were investigated. For large particles composite, multi-domain lead to the introduction of domain wall resonance, which restricts the permeability in low frequency. As the particle size decreases, the particles tend to be single domain particles resulting in a decrease of the domain wall resonance component and an enhancement of the spin rotation one. Two stages with different loss mechanisms were investigated and the sudden increase of the loss near 70 kHz is ascribed to the short circuit of the insulating coating in high frequency due to the space charge polarization. Furthermore, electrochemical analysis also demonstrates the insulation of this soft magnetic composite. This ultrafine FeSiAl composite may promote the development of MHz frequency semiconductor power devices. [ABSTRACT FROM AUTHOR]