Synchrotron small angle X-ray scattering (SAXS) was used to study the supramolecular structure formed by a block copolymer, Pluronic L64 (PEO13PPO30PEO13), in xylene/water mixtures. Lamellar structure was observed at very high polymer concentrations (e.g., C-degree > 0.53 g/mL). The lamellar spacing was determined by the amount of solubilized water and the copolymer concentration, with the amount of water playing a more important role on the lamellar spacing than the copolymer concentration. The lamellar spacing was almost independent of temperature. However the scattering peak became broader with increasing temperature, implying that the micellar size became smaller. Experimental data could be fitted by the Teubner-Strey model, and the resulting periodicity was in good agreement with the lamellar spacing derived by using the Bragg equation.