The origin of the Tsushima Current (TC) is still under debate since direct current observation data are rare and spatial differences in temperature and salinity are too minor to determine the path of the TC. Oxygen isotope is proved to be powerful for tracing the origin of the TC. Oxygen isotope data suggest that there are two water sources of the TC. The TC water could be classified into two water masses with different paths. The depleted oxygen isotope water originates from the Taiwan Strait (T-TC), whereas the enriched oxygen isotope water originates directly from the Kuroshio (K-TC).