Nonequilibrium Green's function formalism can be used to analyze the small-signal ac response of nanoscale devices of one or two dimensional model by introducing internal-induced ac potential. This makes it possible to analyze the small-signal ac response of ultrasmall ballistic MOSFETs. Working on the 2D model of nanoscale double-gate MOSFETs, the present paper calculated the cut-off frequencies and transconductance per current with various channel lengths and gate dielectric constants. It turns out that the cut-off frequencies of ultra-small ballistic MOSFETs will go up without limit with increasing drain currents, which is in contrast to conventional MOSFETs with scatterings. On the other hand, the cut-off frequencies are limited under 3 THz due to the decreasing of transconductance per current. Secondly.. operating at large drain current, the ultra-small ballistic MOSFETs with shorter channels have higher cut-off frequencies at the same drain current, while at small drain current, the longer ones have higher cut-off frequency at the same drain current. (C) 2005 Elsevier Ltd. All rights reserved.