Spatial and temporal variations of hydrography and currents are investigated in the Southeastern Sea of Korea during October 1982. The distribution of the water mass of high salinity (>34.40${ extperthousand}$) and low dissolved oxygen concentration (<5.0ml/l) indicates that the Tsushima current flows northward as it passes the Western Channel of the Korea Strait. The cold water (<$6.0^{circ}c$) with low salinity (<$34.20{ extperthousand}$) and high dissolved oxygen concentration (>6.0ml/l) reaches the bottom of the western channel of the Korea Strait after flowing southward leaning against the slope rather than following the deepest part of the Channel. Repeated sections in the Korea Strait show a remarkable change of hydrographic structure over a period of 4 days ; both warn and cold waters are intensified, particularly in the eastern part of the strait toward the Tsushima Island.