Melt-spun amorphous Fe73.5Cu1Nb3Si13.5B9 (FINEMET, at%) ribbons were ball-milled to obtain powder. The powder was sieved and then passivated in 0.2~1.0% phosphoric acid solution to form a well-coated passivator layer, followed by cold compaction under the pressure of 1.64~1.96 GPa to prepare toroidal magnetic cores. The powder cores were annealed at 400~600 oC to relieve the internal stress and to optimize the crystalline grain size. After annealed at 500 oC for 1 h, α-Fe(Si) crystallites with an average grain size of 14.3 nm were embedded in the amorphous matrix. Combined with the insulation of each powder and the two-phase structure, the powder core exhibits excellent magnetic properties.