Assessment of aging characteristics of conductors and other components of power transmission networks plays an important role in asset management systems. Due to adverse effects of conductor aging caused by annealing, the conductors lose their tensile strength. Although the loss of strength is gradual, it accumulates over time and increases the probability of outages and blackouts. Therefore, the most important factor affecting the strength of transmission conductors is the operating temperature of the line. For this reason, it is important to keep track of conductor temperatures over time, in order to identify segments of power transmission network that may require more close attention, and possibly repairs. This paper describes and illustrates a new methodology for estimating conductor thermal aging using load information and weather conditions derived from historical weather reanalysis, and interpolated to locations of power transmission lines. Conductor temperature is first determined using IEEE 738 standard, and then used to estimate loss of tensile strength in a conductor. The process is illustrated for a single location of a sample transmission line, using assumed load current and historical weather information spanning a period of five years. The simulation results show that the proposed approach provides information vital for transmission asset management and transmission network operating procedures.