The effects of continuous treatment with sugars on the vase life of cut dahlia flowers were investigated. Isothiazolinonic germicide and aluminum sulfate were combined as germicides for sugar treatments. Continuous treatments with glucose at 2.5 or 5% significantly extended the vase life of cut ‘Kokucho’ dahlia flowers and the relative fresh weight was most increased the most by 5%. Treatment with glucose alone, fructose alone, or sucrose alone at 5% or combined treatment with 2.5% sucrose and 2.5% fructose similarly extended the vase life of ‘Kokucho’ dahlia. The combined treatment with sucrose and fructose did not cause petal disorder. Moreover, the combined treatment with sucrose and fructose significantly extended the vase life and increased the relative fresh weight of cut flowers in eight dahlia cultivars.