Characteristics of cylindrical gate-all-around (GAA) SONOS nanowires with elliptical cross sections have been investigated. The mechanism of the program efficiency degradation for the elliptical GAA SONOS is analyzed by 3-D TCAD simulation depending on the geometry aspect ratios (ARs). Moreover, we proposed an effective circular radius ( R _eff1 ) for the elliptical silicon body through a conformal mapping. The ID – V _GS curves of the elliptical and circular GAA SONOS devices with R _eff1 in initial state are almost consistent with each other for AR ≤ 2. However, their program properties differ in less than ~4.2% due to the localized charge-trapping effect in nitride layer of the elliptical geometry.