Localized phase separation was induced inside a glass, the composition of which is not immiscible, by femtosecond laser-induced compositional modification. The glass composition was changed locally from a miscible composition to an immiscible one with high-repetition femtosecond laser irradiation. The phase separation was confirmed by analyzing the composition of the irradiated area with confocal Raman spectroscopy and by observing the co-continuous structure due to phase separation with scanning electron microscopy. The compositional change seems to be related to thermomigration, which is the migration of atoms or ions by the temperature gradient, because the sharp temperature gradient is caused with a high-repetition femtosecond laser. With this method, we can obtain nanoscale co-continuous structure, which would have high surface area, on a glass surface. Moreover, we can control the morphology of the structure by heat treatment while avoiding phase separation in the entire glass because the composition of the non-modified region is not immiscible.© (2010) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.