Radio-frequency application of graphene transistors is attracting much recent attention due to the high carrier mobility of graphene. The measured intrinsic cut-off frequency (f(T)) of graphene transistor generally increases with the reduced gate length (L(gate)) till L(gate) = 40 nm, and the maximum measured f(T) has reached 300 GHz. Using ab initio quantum transport simulation, we reveal for the first time that f(T) of a graphene transistor still increases with the reduced L(gate) when L(gate) scales down to a few nm and reaches astonishing a few tens of THz. We observe a clear drain current saturation when a band gap is opened in graphene, with the maximum intrinsic voltage gain increased by a factor of 20. Our simulation strongly suggests it is possible to design a graphene transistor with an extraordinary high f(T) and drain current saturation by continuously shortening L(gate) and opening a band gap.