In this study, we develop a thermodynamic theory for ideally mixed rodlike micelles. The rodlike micelle is modeled as consisting of two end caps and one cylindrical middle section. A linear model for the end caps’ composition is also proposed to determine how the end caps’ composition varies with the end caps’ size. Also, the thermodyanmic theory is applied to study mixed micelles formed by dihexanoylphosphatidylcholine (diC(6)PC) and diheptanoylphosphatidylcholine (diC(7)PC). The hydrodynamic radius of the mixed micelles is measured by dynamic light scattering for mixed diC(6)PC and diC(7)PC micelles at a total concentration of 25 mM but with various mixing ratios. The measured hydrodynamic radii of the mixed micelles are in good agreement with the values computed according to the thermodynamic theory. Both the theoretical and experimental results demonstrate that the mixed micelles’ mean size initially increases slowly with an increasing percentage of diC(7)PC concentration in the solution. Next, the mixed micelles grow rapidly when the percentage of diC(7)PC concentration exceeds about 50%. Detailed analyses of the mixed micelles by the thermodynamic theory indicate that the cylindrical section of the rodlike micelles on average contains only a few (around five) diC(6)PC molecules for all mixing ratios; meanwhile the number of diC(7)PC molecules in the cylindrical section steadily increases as the percentage of diC(7)PC in the solution increases. The average number of diC(6)PC molecules per micelle in its end caps decreases with an increasing percentage of diC(7)PC in the solution, while the average number of diC(7)PC molecules per micelle in its end caps increases. Moreover, the end caps’ average size steadily increases with an increasing concentration percentage of diC(7)PC.