This work presents a study of AC magnetic susceptibility and the influence of heat treatment on obtaining the nanocrystalline structure for the amorphous alloy of Fe37Co35Nb6B11Si10Cu1 composition. AC magnetic susceptibility at different frequencies and sample lengths, using a variable magnetic field was measured. The influence of heat treatment on obtaining the sample nanocrystalline structure was evidenced by means of XRD technique. The crystallite size, the crystalline volume fraction and the distance between grains for each heat treatment temperature were calculated using the XRD peaks refinement software GSAS. Samples were obtained in the amorphous state by melt spinning technique and all measurements by AC magnetic susceptibility and XRD were performed at room temperature.