The magnetic properties of ultrathin $Fe_{84}B_{9}Nb_{7}$ nanocrystalline ribbon alloy with the thickness of $7-14;mu extrm{m}$ were investigated. It was found that the effective permeability at the frequency over 100 kHz increased with decreasing ribbon thickness. Moreover the core loss decreased considerably with reduction of the ribbon thickness. The effective permeability at 1 MHz and the core loss at 1 MHz and 0.1 T for $Fe_{84}B_{9}Nb_{7}$ alloy with the thickness of $7;mu extrm{m}$ were 3,700 and 2.7 W/cc, respectively. The reduction of thickness to less than $10;mu extrm{m}$ was found to be very effective in obtaining high permeability and low core loss in the MHz frequency range. It was considered that the improvement of magnetic properties in the high frequency range was due to the reduction of the eddy current.