Stability of unduloidlike shapes of a smectic A (Sm-A) phase grown from an isotropic (I) phase has been studied. An equilibrium shape equation for smectic A phase is derived by taking account of the difference in a Gibbs free energy between the smectic A and the isotropic phase, a curvature elastic energy of the smectic A phase, and an interface energy. The stability of unduloidlike shapes has been experimentally examined in the binary mixture of octyloxycyanobiphenyl with dodecyl alcohol. The experimental results can be theoretically explained in case where the experimentally observed surfaces are unduloid of Delaunay’s surfaces.