The microstructures and magnetic properties of electrodeposited Fe-45 wt%Ni-P alloys have been investigated. The structures of electrodeposited Fe-45 wt%Ni alloy was FCC i.e. ${gamma}$ phase and the size of crystallite was 10 nm. The structure of electrodeposited Fe-45 wt%Ni-1 wt%P alloy showed ${gamma}$ phase and 7 nm sized nanocrystalline. The electrodeposited Fe-45 wt%Ni-P alloys containing 2∼3 wt% of P exhibited ${gamma}$ ＋ $alpha$ dual phases. The electrodeposited Fe-45 wt%Ni-P alloys above 3.5 wt% showed an amorphous structure. P in the alloys acted grain refining and phase changing element. The resistivity of the electrodeposited alloys increased with P contents. Effective permeability at high frequency (above 1 MHz) increased with P contents up to 2 wt% and this was ascribed to the easier magnetization rotation owing to the reduction of eddy current. Effective permeability decreased with P contents above 3 wt% and this was ascribed to the transformation of the ferromagnetism of Fe-45 wt%Ni alloy gradually into paramagnetism with the introduction of P into the electrodeposited alloy matrix.