Amorphous ultra-fine particles have been prepared in the systems of FeB and FeMB (M: transition metal) by a chemical reduction method using KBH4 in aqueous solution. The chemical composition of the FeB particles depends on the molar ratio, MR, of KBH4 to Fe2+ ions on mixing. X-ray diffraction patterns show that for MR<9.0 the particles contain a crystalline phase that corresponds to a boron concentration of <30 at.%. The particles have an amorphous structure for values of MR>9.0. The results of Mössbauer measurements show that the particles consist of an amorphous phase, Fe3+ oxide and α-Fe for MR<10.0 and of an amorphous phase and Fe3+ oxide for MR≧10.0. It is also confirmed that the distribution of hyperfine magnetic field, P(Bhf) changes with MR.