Single-crystal iron silicon boron (Fe5Si2B) and iron boride (Fe3B) nanowires were synthesized by a chemical vapor deposition (CVD) method on either silicon dioxide (SiO2) on silicon (Si) or Si substrates without introducing any catalysts. FeI2 and Bl(3) were used as precursors. The typical size of the nanowires is about 5-50 nm in width and 1-20 mu m in length. Different kinds of Fe-Si-B and Fe-B structures were synthesized by adjusting the ratio of FeI2 vapor to BI3 vapor. Single-crystal Fe5Si2B nanowires formed when the FeI2 sublimator temperature was kept in the range of 540-570 degrees C. If the FeI2 sublimator temperature was adjusted in the range of 430-470 degrees C, single-crystal Fe3B nanowires were produced. Fe3B nanowires grow from polycrystalline Fe5SiB2 particles, while Fe5Si2B nanowires grow out of the Fe5Si2B layers, which are attached to triangle shaped FeSi particles. Both the ratio of FeI2 vapor to BI3 vapor and the formation of the particles (Fe5SiB2 particles for the growth of Fe3B nanowires, FeSi particles for the growth of Fe5Si2B nanowires) are critical for the growth of Fe3B and Fe5Si2B nanowires. The correct FeI2 vapor to BI3 vapor ratio assures the desired phase form, while the particles provide preferential sites for adsorption and nucleation of Fe3B or Fe5Si2B molecules. Fe3B or Fe5Si2B nanowires grow due to the preferred growth direction of < 110 >.