The electron mobility in ultrathin-body (UTB) silicon-on-insulator (SOI) metal-oxide-semiconductor field-effect transistors (MOSFETs) with SOI thicknesses from 2.3 to 60 nm is measured in a wide temperature range from 25 to 300 K. In UTB SOI MOSFETs with SOI thickness ranging from 5 to 20 nm, a monotonic decrease of electron mobility with a decrease in SOI thickness is observed at room temperature. At extremely low temperature (25 K), however, the electron mobility of UTB SOI MOSFETs with SOI thickness of 5.7 nm is not degraded but is almost the same as that of thick-body SOI MOSFETs. On the other hand, in UTB SOI MOSFETs with SOI thickness ranging from 3.5 to 4.5 nm, an increase of electron mobility with a decrease in SOI thickness is clearly demonstrated at the effective normal field of around 0.3 MV/cm at 300 K. It is concluded that the decrease of electron mobility in SOI thickness ranging from 5 to 20 nm is due to the increased phonon scattering in thinner SOI films. The observed enhancement of electron mobility with decreasing SOI thickness from 4.5 to 3.5 nm is ascribed to the subband structure modulation induced by quantum confinement effects in nanoscale SOI films. The impact of SOI thickness fluctuations on electron mobility and the methods to evaluate the thickness of ultrathin SOI films are also discussed.