The aggregate forms of bile salt and lipid mixture were studied by a statistical mechanical theory. The free energies of disclike mixed micelles, cuplike vesicles, and of rodlike micelles were defined and the minimized free energies were compared to study the stable forms of these aggregates. In addition to the adsorption affinity free energy of bile salt to the vesicle membrane, the following two energy terms were taken into account: one is the bending energies of the cuplike vesicle and,rodlike micelle, and the other is the enhancement of the affinity free energies of bile salt from aqueous solvent to the curved membranes. By postulating the spontaneous curvature, which is assumed to be proportional to the surface coverage of bile salt, the cuplike vesicle and the rodlike micelle were shown to be mechanically frustrated. Despite the bending energy cost, the cuplike vesicle was, shown to be the stable aggregate form in a certain concentration region. In this concentration region the molecular weight of the aggregate is expected to increase monotonically, while the radius of gyration is expected to possess a maximum value. This is consistent with the light scattering measurement made by Egelhaaf and Schurtenberger (J. Phys. Chem. 1994, 98, 8560).