We established a regional ionospheric model for investigating ionospheric TEC (Total Electron Contents) variations over the Korean Peninsula during major geomagnetic storms. In order to monitor the ionospheric TEC variations, we used nine permanent GPS reference stations uniformly distributed in South Korea operated by the Korea Astronomy and Space Science Institute (KASI). The cubic spline smoothing (CSS) interpolation method was used to analyze the characteristics of the ionospheric TEC variations. It has been found that variations of TEC over the Korean Peninsula increase when a major geomagnetic storm occurred on November 20, 2003. The TEC has increased about one and a half of those averaged quite days at the specific time during a geomagnetic storm . It has been indicated that the KASI GPS-derived TEC has a correlation with the geomagnetic storm indices (eq. Kp and Dst indices).