Through an idealized modeling experiment, in which the East Sea is simplified as a small rectangular basin with a through flow placed at the upper left corner of a larger basin, the effect of the Tsushima Warm Current on the East Sea has been investigated. The NOAA/GFDL MOM V.3 is used. The horizontal resolution is 0.25° both in the zonal and meridional directions. The basin is 4000 m deep, and there are 36 depth levels in the vertical. The effects of surface wind stress are also considered by comparing results with or without a zonal wind stress. As well known, the current supplies heat to the East Sea and when the inflow is considered, the model ocean becomes warmer. The water flowing from the small basin into the larger basin is cooler than the western boundary current and colder water is found near the outlet in the cases with the through flow. The inflow also brings variability from the larger basin into the smaller basin, and the cases with the through flow show greater variability. The model is idealized and we may not be able to apply model results to the East Sea directly, but the model results show that through the Tsushima Warm Current the variability in the Northwestern Pacific could be transferred to East Sea.