A bicelle is a non-spherical, bilayer molecular assembly that has potential application as a functional membrane material. In this study, we examined the effect of molar ratio in preparing bilayer molecular assemblies from oleic acid (OA) and a detergent (CHAPSO) by evaluating the size and the membrane packing of the prepared molecular assemblies. As a result of the measurement by dynamic light scattering, the size of the solubilized molecular assemblies could be controlled in the range of 22.5-135 nm by varying the molar ratio of OA, XOA (=[OA]/([OA]+[CHAPSO])), in the range of 0.3-0.7. The fluorescent probe method employing Laurdan revealed that the membrane packing of the solubilized assemblies prepared from OA and CHAPSO was heterogeneous, and that a bilayer structure containing bicelles was probably formed. Moreover, it was found that addition of Na+ as a counterion improved the dispersion stability of the bilayer molecular assembly prepared.