M agnetic anisotropy of a nanocrystalline Fe73.5Cu1Nb3Si13.5B9 soft magnetic alloy was investigated. Theoretical analysis and measurement results show that the effective magnetic anisotropy constant in the nanocrystalline alloys can be measured by means of the process of approaching saturation and the measured value is about one orders of magnitude smaller than the magnetocrystalline anisotropy constant K1 of the α-Fe(Si) grains.