We have systematically investigated electron mobility behaviors in germanium-on-insulator (GeOI) metal-oxide-semiconductor field-effect transistors (MOSFETs) with reducing the Ge channel thickness down to 9 nm. 9 nm-thick GeOI n-MOSFETs operated with a reasonable Ion/Ioff ratio of ∼105, but it showed the electron mobility degradation compared with thick GeOI case. To investigate the physical origin of the mobility degradation in ultrathin body (UTB) GeOI n-MOSFETs, the depth profiling of GeOI crystallinity was investigated by Raman spectroscopy. A difference of Ge crystallinity in the front channel from that in back one was discussed to explain the mobility degradation in UTB region.