As Global Positioning System is able to provide 24-hour all weather surveying capability and high precision survey in three dimension, expected that the extensive use of GPS to support geophysics, geophysics, millitary and time correction etc. But in order to use the GPS results effectively, we have to solve problems about coordinates transformation relating the WGS84 to Bessel Datums and development of the accurate geoid undulation model. In this paper, we derive polynomial model equations about geoid undulation around local area(longitude $126^{circ}{sim}129^{circ}$, latitude $36^{circ}{sim}37^{circ}$) in Korea by GPS/Leveling method, also study the geoidal height calcaulation methods supplemented by Earth Gravitational Models (OSU981A, OSU86F).