We describe a new approach to the internal refractive index modification of glass by a femtosecond (fs) laser. The glass we used is a photosensitive glass Foturan which contains trace amounts of silver. Silver nanoparticles, which is responsible for the refractive index change, can be formed in the glass after exposed to the fs laser and then postbaked at an appropriate temperature between 500°C and 550°C. In this work, latent images of grating structures are first inscribed into the photosensitive glass by photochemical reaction of a tightly focused fs laser beam with an intensity much lower than the threshold of optical breakdown. After this step, no measurable diffraction can be observed by irradiating the gratings with a He-Ne laser beam. The samples are then baked at 520°C for various durations from 3h to 18h. Diffraction of the optical grating embedded in the glass can now be observed, and the diffraction efficiency increases with postbaking duration, indicating that a refractive index change occurs in the modified regions. The relationship between the refractive index change and the postbaking duration is systematically investigated.