An accurate measurement and understanding of horizontal stress rates within the earth's crust is important for providing fundamental insights into the mechanisms driving plate motion and intraplate tectonic activity. In this paper, the stress field within the crust of the South Korean peninsula is obtained from observed displacement rates based on 4-year continuous GPS observations (2000 - 2004). Results show that the South Korean peninsula is under both compression and extensional stress regimes with maximum shear stress rate being in the mid part of the study area coinciding with the geologically defined Honam Shear Zone (HSZ) and relatively highly seismically active zone. In addition, the variation rate of strain energy density, an important index reflecting the intensity of crustal activity, is further derived, and its distribution indicates high earthquake potential in the mid-part as well as the north and northeast edges of the crust of the South Korean peninsula.