We investigate the effect of thermal aging on the optical properties of multimode silica fibers. Samples are submitted to several heating-cooling cycles of 8 h each. Analysis is performed using the optical spectrum analyzer, the fixed analyzer method, and the polarization-dependent loss measurements to determine respectively the signal loss values, the differential mode delay, and the polarization-dependent loss. Results show an increase of the attenuation coefficient, the differential mode delay, and the polarization-dependent loss values with the increase of aging time. The obtained results show a degradation of the optical properties of fibers from 1680 h related to structural changes, and to the increase of the optical anisotropy of fibers.