The iron di-silicide β-FeSi2 is a promising direct band gap semiconductor but difficult to produce. Here, the successful direct synthesis of this phase by ion beam mixing of Fe/Si bilayers at temperatures in the range of 450 to 550°C is reported. The obtained single-phase β-FeSi2 layers and their structure are confirmed by Rutherford backscattering spectrometry, X-ray diffraction and conversion electron Mossbauer spectroscopy.