Investigations on unidirectional anisotropy and rotational hysteresis loss of exchange coupled Ni81Fe19/NiO films have been conducted to clarify the nature of the exchange coupling mechanism. The interfacial exchange coupling regions, which had been considered to be scattered among the nonexchange coupling regions matrix, were found to be composed of many local regions of two kinds: (i) blockable regions which can give the Ni81Fe19film a unidirectional anisotropy and (ii) unblockable regions which can have exchange coupling, but cannot give the Ni81Fe19film unidirectional anisotropy. These unblockable regions begin to change gradually to blockable regions on decreasing the temperature below around 100–110 K. This change is probably caused by the antiferromagnetic NiO anisotropy of unblockable regions being strengthened below that temperature. Moreover, the decrease in size of the exchange coupling field and lowered blocking temperature fortNiO<50 nm (tNiO:NiO film thickness) seems to originate from a decrease of antiferromagnetic NiO anisotropy with decreasingtNiO. ©1995 American Institute of Physics.