Structural modification of iron-disilicides beta -FeSi2 has been investigated to improve the thermoelectric properties of the materials by r.f.-plasma processing in SiH4, GeH4 or their mixed gases. The plasma treatment of beta -FeSi2 micrograins in SiH4 or GeH4 gas prior to sintering results in an anomalous increase in the hole mobility in the temperature region lower than 400 K. X-Ray photoemission spectroscopy (XPS) measurements revealed that the valence states at the interface between a coated Si or Ge layer and a beta -FeSi2 core material are changed by the interdiffusion of Si into beta -FeSi2 for SiH4 plasma and by the out-of-diffusion of Ge from the base material for GeH4 plasma. From electron paramagnetic resonance (EPR) measurements, it is also found that the carrier mobility at temperatures lower than 400 K is limited by structural defects at the grain-boundary interface, and that the defects are greatly reduced by the r.f.-plasma treatment.