A maximum magnetoimpedance (∼66%) was observed at 5.2 MHz in a positive magnetostrictive glass-coated amorphous Co83.2Mn7.6Si5.8B3.3 microwire. Giant magnetoimpedance in this microwire was increased to 129% on short-duration heat treatment by passing four current pulses of amplitude of 100 mA and each of 12 s duration. Magnetoimpedance of the as-quenched and heat treated microwires was investigated as a function of a dc magnetic field Hdc within ±120 Oe and frequency f up to 12.85 MHz. The magnetization measurements of the samples indicated that the increase in giant magnetoimpedance on short-duration annealing is due to the increase in outer shell volume of the domain structure.