The A-B-type silicone copolymer, Me3SiO-(Me2SiO)(23)-Me2SiCH2CH2CH2-O-(CH2CH2O)(51.6)H (Si25C3EO51,6), forms only lamellar liquid crystal (L-alpha) in water over a whole range of concentrations. Rich phase behavior was observed upon addition of conventional nonionic surfactant, C12EO5, whose molecular weight is similar to1/10 of Si25C3EO51,6. For both amphiphiles, f, the volume fraction of hydrophilic part in amphiphile, is the same, and f = 0.5. C12EO5 alone forms aqueous micellar solution (W-m), normal hexagonal (H-1), bicontinuous cubic (V-1), lamellar (L-alpha), and reverse micellar solution (O-m) phases in water with increasing surfactant concentration. On the other hand, W-m, discontinuous micellar cubic (I-1), L-alpha, reverse bicontinuous cubic (V-2), and reverse hexagonal (H-2) phases are successively formed in water at a 70/30 weight ratio of Si25C3EO51.6/Cl2EO5. A reduction in the effective cross-sectional area per amphiphile at hydrophobic surface (a(S)) takes place upon addition of C12EO5 in the mixed system. A small amphiphile, C12EO5, is dissolved in the copolymer L-alpha phase, and a relatively small amount of surfactant occupies a rather large area at an interface of aggregates, although the as for Si25C3EO51.6 in the L alpha phase is more than double that for C12EO5. Hence, the surfactant mainly dictates the morphology or amphiphile-layer curvature in the mixed system. The copolymer is practically insoluble in the L-alpha phase of C12EO5 due to the packing constraint. Hence, two La phases coexist in a surfactant-rich region at Ws = 0.75, where Ws is the weight fraction of total amphiphile in the system. On the other hand, the copolymer L-alpha phase changes to the I-1 phase with increasing the surfactant mixing fraction at W-S = 0.55. The spherical micelle in the I-1 phase has a double-layer structure in which the surface is covered by surfactant and the core is pure poly(dimethylsiloxane) chain.