Soft magnetic properties of Co-based amorphous alloy of the composition Co 67 Fe 4 Ni 2 Si 15 B 12 have been investigated by isothermal heat treatment up to the conventional crystallization temperature. In the as-cast condition the Curie temperature of the sample is 272°C and saturation magnetization is 74emu/g. Magnetic properties undergo variation depending on the heat treatment temperature. For the heat treatment temperatures of around 420 and 490°C, superior soft magnetic properties are obtained. For both the temperatures initial permeability, μ′ reaches value up to ten times the value of permeability in the as-received samples. Annealing effect on giant magneto-impedance has been observed for the current-driving frequencies of 4.5 and 6MHz. Field dependence of magneto-impedance shows hysteresis at low field, which is related to the changes in the magnetization process of the sample.