The evolution of microstructure, magnetic and magnetoimpedance (MI) properties on heat treatment in Co-rich amorphous alloys with small quantities of 3d or 4d elements is presented. Magnetic properties and magnetoimpedance of the ribbons are investigated with replacement of Ni and Mn by Nb. It is found that the microstructure and magnetic properties can be tuned when samples are annealed below crystallization temperature. From the present studies it is observed that microstructure evolved in the case of Co73Fe4.5Nb1.5Si4.2B16.8 alloy composition appears to be suitable for obtaining higher MI values and magnetic field sensitivity. Further it is also found that the presence of Nb inhibits the anisotropic grain growth and results in better MI values compared to the compositions studies in the present investigation.