The crystallisation process and the ultras-soft magnetic properties of amorphous/nanocomposite alloy Fe73.5Si17.5B5Nb3Cu1 fabricated by conventional melt-spinning technique are systematically investigated in terms of thermal analysis and in-situ measurement of magnetisation dynamics. The thermal analysis using differential scanning calorimetry showed that crystallisation from Fe-based amorphous state to α-Fe(Si) started at 535∘C. Further heating the sample leads to a transformation from the α-Fe(Si) to Fe-B phases at 670∘C. Crystallisation activation energies were determined using two models: Kissinger and John–Mehl–Avrami (JMA), which were consistent to each other with a value of 2.81±0.03eV. High resolution transmission electron microscopy investigation revealed an ultrafine structure of α-Fe(Si) nanocrystallite with mean size of 12.5nm embedded in an amorphous matrix. At a volume fraction of 86% of α-Fe(Si) phase, optimum soft magnetic properties were obtained with very high permeability of 110,000 and a very low coercivity of 0.015Oe by annealing the amorphous alloy at 530∘C in 40min.