The segregation of non-magnetic phases such as borosilicate and Cr was investigated by crystallization behavior during the surface and bulk crystallization of Co-based amorphous alloys. The concentration of metalloids (B and Si) determined the extent of grain boundary segregation of borosilicate glass during surface crystallization. During the bulk crystallization of (Co75Cr25)0.8Si5B15 amorphous alloy, solute rejection of Cr resulted in the nucleation of Cr-deficient ferromagnetic crystals and non-magnetic σ-phase was subsequently precipitated along the grain boundary. These results show that crystallization process, i.e. nucleation and growth can be controlled to optimize the microstructure to reduce media noises in Co-based recording media.