Abstract: Microstructures of differently annealed nanocrystalline Fe_73.1Cu_1.2Nb_3.2Si_12.5B₁₀ alloy have been investigated by means of Mössbauer spectroscopy and TEM at room temperature. The values of volume fraction and average diameter of α-Fe（Si） crystallites as well as approximate average thickness of the intergranular layer of the amorphous matrix have been given. Temperature dependences of a.c. initial susceptibility of differently annealed samples during heating have carefully been examined. It is shown that the susceptibility first decreases near-linearly with increasing heating temperature above the Curie temperature of amorphous matrix, reaches a minimum value at temperature clearly higher than the Curie temperature of the amorphous matrix, and then increases. This temperature behaviour has been qualitatively interpreted by using the phenomenological theory proposed by Hernando et al.