Temperature dependence of initial permeability is investigated for nanocrystalline Fe38.4Co40Si9B9Nb2.6Cu alloy annealed at 500 and 600℃,and the initial permeability of 600℃-annealed sample is observed not to drop sharply at the Curie temperature of the residual amorphous phase,which is a new magnetic phenomenon in dual-phase nanocrystalline alloys. The origin of the above phenomenon is explored by estimating the Curie temperature of amorphous ribbons which have the same compositions with the residual amorphous phase in annealed nanocrystalline alloys. The results indicate that the Curie temperature of the intergranular amorphous region can be enhanced drastically up to the Curie temperature of the crystalline phase (TAC=TαC) when the exchange-field between adjacent nanograins penetrates through the amorphous interphase thoroughly. Furthermore, the effective exchange penetration length of FeCo-based nanocrystalline alloys (LFeCo) is evaluated to be 0.61 nm much larger than that of Fe-based nanocrystalline alloys, which may be the main reason of the higher permeability of FeCo-based alloys at elevated temperature.