We investigate the effect of thermal aging on the optical properties of multimode silica fibers. Samples were submitted to several heating-cooling cycles of each. Analyses were performed using an optical spectrum analyzer and an optical time-domain reflectometer to determine the signal loss. We proved that the attenuation coefficient is closely related to Rayleigh backscattering. Results show an increase of the attenuation coefficient with aging time. The obtained results show a degradation of the optical properties of fibers from on, due to local variation of the refractive index induced by fluctuation of the material density.