We have investigated the effect of symmetry on the fluctuation of a three-component lamellar phase composed of oil (octane), water, and a nonionic surfactant (C12E5). The studied lamellae consist of various oil/water volume ratios for a fixed surfactant concentration. From aligned lamellae, we succeeded in obtaining highly reproducible, high-resolution X-ray spectra with very small full width at half maximum (6 x 10(-4) Angstrom(-1)). The deviation of the periodicity expected by the dilution law can be explained from the fluctuations. Measuring this deviation, we observed that for the constant surfactant concentration, lamellae with equal oil/water ratios have higher fluctuation than when oil/water --> 0 or infinity because of the difference of the flexibility of monolayers and bilayers.