A micellar surface charge modification for the regular solution theory was performed by considering the counterion bind effect. For mixed micelles of cationic fluorocarbon-nonionic hydrocarbon surfactants, this modified equation can be tested by measuring the mixed cmc, surface potential, aggregation number, and molar volumes. The mixed cmc curves are well fitted with the calculated curves by the modified equation as a function of cationic surfactant molar fraction. The interaction between the surfactants is not so repulsive as expected from the regular solution theory. In addition, the mixed micelles with various mole fractions exist in a wide mixing region and the size of mixed micelles having charge is fairly small.