As the deepest channel in the central Ryukyu Island arc, the Kerama Gap plays a crucial role in the water exchange between the Northwestern Pacific and the East China Sea. The Kerama Gap has a width of ~50 kilometers and a maximum sill depth of ~1050 meters. Although the mean volume transport of Kerama Gap is just 2.0 Sv (), its fluctuation is quite large, with a standard deviation of ~5.0 Sv, which is thought to have a significant impact on the western boundary current system in the North Pacific Ocean, mainly composed by the Kuroshio current and Ryukyu current. Based on the altimeter data and an eddy-resolving ocean general circulation model (OFES model), we found a dramatic difference between the upstream and downstream transports bordered by the Kerama Gap. A strong current core at ~500 m dominates the variability of volume transport through the Kerama Gap, which can be well described by one classical two-layer rotating hydraulic theory. In addition, the variability in the deep overflow via the Kerama Gap is highly correlated with the density anomaly at 500 m on the Northwestern Pacific side, and the density anomaly signal is found mainly from the open ocean in the east.