In this paper, the on state quasi-saturation effect of high voltage SJ-VDMOS (Superjunction vertical double diffused MOSFET) which is formed using alternate process of boron implantation and n-epitaxial growth for several turns are investigated in detail. Transconductance of the device is studied by simply formula derivation of MOSFET. Velocity saturation in the JFET region is strongly influence the transconductance performance of the device. In order to improve the quasi-saturation and have a flatter transconductance, an improved structure having floating p-pillar is proposed without additional process. Eventually, the above results are confirmed by simulation and measurement.