For two expression forms of Frequency-dependent Equivalent (FDNE), i.e. impedance and admittance, this paper proposes a method which transforms between impedance and admittance forms based on state space description. To achieve the transformation, the state space equation of impedance or admittance is firstly divided into differential part and the remaining part; and then these two parts are separately transformed into another form based on state space description. Last, validity is verified by example. It provides a method achieving the transformation of FDNE forms between impedance and admittance based on state space description. This work is supported by Scientific and Technology Project of China Southern Power Grid (No. KY2015-2-0002).