The kinetics of the crystallization of Fe73.5Cu1Nb3Si13.5B9 amorphous alloy was studied using Mossbauer spectroscopy and X-ray diffractometry. During isothermal annealing of the samples, two phases were observed : a crystalline D0(3)-FeSi alloy with fine grains and an amorphous phase enriched with niobium, boron and copper. It was found that the growth rate of the particles of the crystalline alloy was controlled by diffusion. For longer annealing time or at higher annealing temperature the growth process was found to be suppressed, probably by the niobium atoms. The activation energy obtained for the crystallization was about 143 kJ mol-1.