Aggregation behavior of hydrophobically modified ethyleneglycol oligomer, poly(ethylene glycol) monododecyl ether (C12E25), in aqueous solution has been studied in wide ranges of concentration and temperature by means of light scattering, X-ray scattering, and linear-viscoelastic measurements. C12E25 forms stable micelles in dilute solution. It is found that the micellar solution exhibits the sol-gel transition accompanied with disorder-to-order structural transition as increasing concentration at lower temperatures, or with decreasing temperature at higher concentrations. The transition concentration is located around 30 wt% irrespective of temperature, while the transition temperature is about 56 C independent of concentration. In the gel region, the micellar particles form a long-range order structure of body-centered cubic lattice.