Soft magnetic alloys consisting of nanoscale fcc Fe grains have been developed by primary crystallization of melt-spun amorphous alloys as typically exemplified in Fe-B, Fe-M-B (M = Zr, Hf, Nb) (Nanoperm)  and Fe-Si-B-Nb-Cu (FINEMET)systems. Lot of scientific effort has been put in last years to control the crystallization process of metallic glasses. This is due to the fact that several attractive properties of the resulting material are strongly related to the final attained microstructure. In the present paper, primary nanocrystallization kinetics of a Fe–based multicomponent amorphous system namely Fe67Co18B14Si1 (2605CO) has been analysed by nonisothermal DSC measurements. Crystallization is a combined process of nucleation and growth. The amorphous alloy undergoes two-step crystallization with primary crystallization of α-Fe giving the first step. The Avrami exponent for the two steps has been derived. A detailed analysis of the data provides an insight in to the dimensionality and mechanism of growth.