In this paper a presentation of the nanocrystallisation process on Fe73.5Cu3Nb1Si13.5B9 rapidly solidified and glass covered wires is given, through Giant Magneto-Impedance (GMI) and low field magnetic susceptibility measurements. Conventional (furnace) thermal treatments and de joule heating were performed on selected samples of both rapidly solidified and glass covered wires. Low field magnetic susceptibility measurements were also made to more completely describe the behaviour of the studied systems. Rapidly solidified and glass covered wires both show a good GMI response (of the order of 100 %), at least at frequencies of 5-10 MHz. High GMI values have been obtained also for joule heated samples, but because of their low reproducibility they are not clear indicators of the nanocrystalline state of the material.