Lateral asymmetric MOSFET, which has longitudinal doping variation in the channel, is the core of high voltage MOSFET. Recently it has been recognized that capacitance property of this kind of device is fundamentally different from conventional MOST because Ward-Dutton (WD) charge partitioning is not applicable to this kind of devices (Aarts, 2006). In this work we show the existence of a partitioning scheme for small-signal operation of the device. We also provide physical explanations of unusual behavior of C dg in lateral asymmetric MOST. The proposed theory is validated by extensive numerical and device simulation.