Abstract: A Study on the utilization of the ratio of K content between 5th and 1st leaves (v m /Im) for K nutrient diagnosis was carried out in 1986~1991. It was found that the K content tested by routine method with 3rd blade greatly varied caused by many factors, but comparatively smaller by K supplement. The most sensitive part to K ftertilization was in the mid-upper midrib at low positions. The relative value of K content, expressed by Vm/Im, in the midrib between 5th and 1st leaves from top, was used for evaluating the K nutrient status of banana. The sampling part was in the connective zone between midrib and blade. The feasiblity of testing by mixture samples was defined and three factors affecting extraction rate were compared. Based on the above mentioned results, the index Vm/Im was set, the value 1.0 as K defficiency, 1.0 as sufficiency. This method offered a longer period for sampling than 3rd blade did and eliminated the trouble caused by the difference of K content among the varieties, and make the application more convenience. Vm/Im value had significant possitive correlation with fresh-fruit yield.