In the ranges of f=1020000Hz and Bm=0.11.0T, the core loss has been measured for nanocrystalline Fe73.5Cu1Nb3Si13.5B9 alloy with high Br. The total power loss was decomposed into the sum of the classical eddy current loss, the hysteresis loss and the excess loss. The results show that the change of the total loss per cycle with frequency clearly shows a nonlinear behavior, which has been described in terms of Bertotti's statistical theory of loss. The frequency range examined can nearly be divided into two intervals, in which there exist two different describing ways.