An amphiphilic graft copolymerwas obtained using a macromonomer method and its aggregate behavior and phase structure in aqueous solutions were studied by tensionmetry, polarized micrography, rotation rheometry and small-angle X-ray diffraction. Firstly, the graft copolymer PBA-g-PtBMA was synthesized by a free radical copolymerization of butyl acrylate (BA) with poly(t-butyl methacrylate) (PtBMA) macromonomer, which has the vinyl end group and was prepared by a catalytic chain transfer polymerization using bis[(difluoroboryl)diphenylglyoximato] cobalt(II) (COPhBF) as the chain transfer catalyst.Then PtBMA segment in thegraft copolymer PBA-g-PtBMAwas quantitatively hydrolyzed under an acidic condition and neutralized by NaOH solution to prepare the amphiphilic graft copolymer PBA-g-P(MAA－Na＋) with hydrophobic backbone and hydrophilic side chains. The surface tension of aqueous solutions with different concentration of amphiphilic copolymer was measured by Wilhelmy plate method. The plot of surface tension vs. concentration was different from that of typical surfactants. The aggregate diameters measured by a dynamic light scattering method haddouble peaks (with peak values of 30 and 300 nm) and kept nearly constant during the concentration from 0.02 to 10 g/L. Polarized micrograph of the amphiphilic copolymer solution (w＝37.5%) showed the typical lamellar liquid crystalline pattern. And the concentrated solution exhibited distinct viscoelastic property. The interlayer space (about 12.6 nm), was measured by small-angle X-ray diffraction.