The micellization of PS-PEO diblock copolymers has been studied in water-organic mixed solvents comprising water-THF and water saturated with toluene by laser light scattering techniques. THF and toluene was absorbed into the micelle core when the slight amount of the organic solvents was added to an aqueous micellar solution. As consequence, the aggregation number of the micelle is increased. However the aggregation number of the micelle decreases as increases the volume fraction of THF due to the reduction of interfacial tension between the core and water-THF mixed solvent. Experimental data agree with the theoretical prediction based on the minimization of the free energy including interfacial energy. In conclusion, interfacial energy is an important parameter determining the aggregation number of the diblock copolymer micelles in a water-organic mixed solvent.