Loading profiles and thermal transient models of XLPE-insulated underground cables are analyzed for thermal aging evaluation of the cable. Since the underground cables have limitations in accessibility, observability, and application of diagnosis techniques, it is crucial to assess the cable condition and to determine appropriate replacement time. The thermal aging of power cables is mainly caused by heat loss on its conductor by the load current. Therefore, in this paper, real-world load profiles of an XLPE insulated underground cable are used to assess the thermal aging. With the load profiles, the thermal transient models are used to estimate the conductor temperature. The estimated temperatures are applied to the Arrhenius model for thermal aging evaluation. For three different load profiles, the results of the thermal aging assessment are compared and the results show the relation between average loads and thermal aging. The proposed technique will enable us to determine the proper replacement time of the underground cable in terms of the thermal aging.