The development of third-generation semiconductors has increased power density in electronic devices while increasing demands for high-frequency performance of internal soft magnetic materials. Fe-based nanocrystalline alloys are among the most promising candidates for high-frequency applications owing to their excellent comprehensive soft magnetic properties, including high saturation magnetic flux density (B_s), high permeability, and low core loss per unit mass (P_cm). However, further improvements in their high-frequency properties are required. This study prepared Fe_75.2Si₁₃B₈Cu₁Nb_2.8 nanocrystalline alloy ribbons with 15-23 μm thicknesses by adjusting the Cu wheel speed. The effects of ribbon thickness on the structure and static/high-frequency magnetic properties of the nanocrystalline alloys were investigated. Furthermore, the high-frequency magnetization mechanisms of nanocrystalline alloys with varying ribbon thicknesses were examined through magnetic domain structure characterization. Results indicate that all as-spun alloy ribbons exhibit an amorphous structure and transform into a similar amorphous + α-Fe nanocrystalline dual-phase structure after annealing at 843 K for 60 min, with average α-Fe grain sizes of 11.0-11.6 nm. The static magnetic properties of all ribbons are nearly identical, with B_s and coercivities of 1.35-1.36 T and 0.5-0.6 A/m, respectively. On the contrary, the high-frequency soft magnetic properties improve with decreasing ribbon thickness. The effective permeability (μ_e) of thinner ribbons remains stable and exhibits milder attenuation with increasing frequency. At 100 kHz and 1 MHz, the 15-μm ribbon has μ_e of 17000 and 5200, respectively, which are substantially higher than the values of 14000 and 2900 for the 23-μm ribbon. Moreover, the thinner ribbons demonstrate reduced P_cm. At 0.2 T, 100 kHz and 0.2 T, 500 kHz, the 15-μm ribbon shows P_cm of 67 and 811 W/kg, representing reductions of 38.0% and 41.9%, respectively, compared with the 23-μm ribbon. Loss separation analysis indicates that the reduced P_cm of the thin ribbon is primarily attributed to decreased eddy current loss and residual loss. The decreased ribbon thickness refines the magnetic domains, facilitating domain rotation at high frequencies and improving high-frequency permeability while reducing residual loss.