Abstract: Improving high-frequency magnetic stability of Fe-based nanocrystalline materials in long-term thermal environment is important and necessary for the reliable and functional application of power electronic devices. The effects of thermal-environment time on the permeability, core loss, phase structure and domain structure of FeCuNbSiB and FeCuNbSiBMo nanocrystalline cores with saturation magnetic induction of 1.38-1.39 T were studied by means of AC B-H,impedance analyzer, X-ray diffractometer（XRD） and magneto-optical Kerr microscope. The results show that the both nanocrystalline cores have good stability in variation of high-frequency permeability with thermal-environment time under temperature of 130 ℃. The relative variation rate of permeability of both alloys under two thermal-environment times at room temperature decreases with the increase of frequency. The Mo-free core exhibits much smaller variation of permeability with thermal-environment time than that of the Mo-added core. Furthermore, the influence of thermal-environment time on the core loss of both cores is very small, and and the high frequency magnetic stability of the Mo-free core is better. Microstructural and domain structure characterization suggests that 720 h time does not change the phase structure, but change the magnetic domain of both cores, which is the reason for the change of magnetic properties.