We present the magnetic characterization of nanocrystalline amorphous ribbons with nominal composition Fe73.5B9Si13.5Mo3Cu1, obtained by melt spinning. An annealing treatment was made on the alloys during 10, 20, 40, 50, 60 and 70 minutes at 400°C, in order to induce a nanocrystalline phase. Measurements of magnetization were made to evaluate the coercive field (HC) as a function of annealing time. Results of vibrating sample magnetometry (VSM) show two important changes: a decrease in HC value as annealing time increases, until reaching a minimum value of 31 Oe, corresponding to the amorphous ribbon with annealing time of 20 minutes. This behavior indicates a soft magnetic state suggesting a structural relaxation. At this time, it is observed the best magnetic properties. In the second change, HC increases until reaching a maximum value of 34 Oe, corresponding to 60 minutes. This increase indicates the onset of nanocrystallization state for the FeMo phase. In order to evaluate the magnetic anisotropy field (HK), measurements of low field absorption (LFA) were carried up by means of an electronic paramagnetic resonance (EPR) spectrometer modified. The peak-to-peak linewidth (ΔHPP-LFA) of LFA signal is associated with HK, therefore HK can be evaluated during the different annealing times.