High-density of arrays of self-oriented boron nanowires grown on silicon substrates were synthesized by radio-frequency magnetron sputtering with a target of highly pure boron and boron oxide mixture using argon as the sputtering atmosphere. TEM studies show that the conventional growth mechanisms such as Frank screw-dislocation mechanism and the vapor–liquid–solid mechanisms are not suitable for the one-dimensional growth of boron nanowires. The oxide-assisted cluster–solid mechanism for the Si and Ge crystalline nanowires is not completely suitable for our case. The vapor–cluster–solid mechanism is proposed for the well-aligned growth of the amorphous boron nanowires.