PFS-Fuzzy classification ( Lu, 1989) was used on observational data obtained during a cruise (July-August> 1987)to classify the water masses in the source area of the Tsushima Warm Current. Their mixing features were studied by using numerical index analysis of fuzzy sets. The calculated results showed there are nine water masses belonging to three basic types.The analyses suggest that, though, in summer, the Surface Water of the Tsushima Warm Current located in a strongly mixed area is a mixture of the East China Sea Mixed Water, the Kuroshio Surface Water and the Kyushu Western Coastal Water, it originates mainly from the Kuroshio Surface Water and its deep water comes from the Kuroshio Subsurface Water. This study reveals that 1) regions such as the intensely mixed region, the frontal zone and the transition zone, Water, it originates deep water comes from water, usually have a higher fuzzy degree ; 2) water masses with higher stability and little modification have a lower fuzzy degree ; and 3)mixed PFS-Fuzzy classification (Lu, 1989) was used on observational data obtained during a cruise (July-August> 1987) to classify the water masses in the source area of ​​the Tsushima Warm Current. Their mixing features were studied by using numerical index analysis The calculated results showed there are nine water masses belonging to three basic types. the analyzes suggest that, though, in summer, the Surface Water of the Tsushima Warm Current located in a strongly mixed area is a mixture of the East China Sea Mixed Water, the Kuroshio Surface Water and the Kyushu Western Coastal Water, it originates mainly from the Kuroshio Surface Water and its deep water comes from the Kuroshio Subsurface Water. This study reveals that 1) regions such as the intensely mixed region, the frontal zone and the transition zone, Water, it originates deep water comes from water, usually have a higher fuzzy degree; 2) water masses with higher stability and little modification have a lower fuzzy degli ree; and 3) mixed