For the first time, the small-signal model of MOSFET cascode with merged diffusion is presented. It is the cascade of the two quasi-static MOSFET small-signal equivalent circuits. Drain of one transistor and source of another transistor is shared with merged diffusion. By Y-parameter analysis, capacitances, resistances and transconductances comprising the small-signal equivalent circuit were extracted analytically using four port S parameter measurement. This modeling method was verified with the measured Y parameter data up to 15 GHz.