Aluminium Conductor Steel Reinforced (ACSR) is a key component in high-voltage power transmission networks due to its combination of excellent electrical conductivity from aluminium and mechanical strength from the steel core. However, during long-term operation, ACSR undergoes mechanical property degradation due to repeated exposure to high temperatures (thermal aging), potentially reducing the reliability of the transmission system. This study aims to analyze the effect of high-temperature treatment on the ultimate tensile strength of new and used ACSR conductors, as well as to model their service life prediction using the Arrhenius method. Experimental testing was conducted at PT PLN Low Voltage Laboratory, Certification Center, during June–July 2025 using the accelerated thermal aging method at temperatures of 90°C, 100°C, 110°C, 120°C, and 130°C for 24 hours and 120 hours, followed by tensile testing with a Universal Testing Machine according to ASTM B557. The test data were analyzed using the Arrhenius method to estimate the predicted service life of new and used conductors. The results show that within the temperature range of 75°C–130°C, the service life of new conductors ranged from 42.4 years, while used conductors lasted only from 0.97 years to 0.014 years (5 days). Increasing the temperature from 75°C to 130°C reduced the service life by approximately 99.72% for new conductors and 98.56% for used conductors. The difference in service life at higher temperatures is significant; for example, at 100°C, new conductors lasted approximately 3.86 years, while used conductors lasted only about 0.085 years (1 month). The degradation rate increased exponentially with rising temperature, confirming that high operating temperatures are the dominant factor accelerating ACSR deterioration. These findings provide a technical basis for assessing the feasibility of reusing used conductors and developing asset management strategies for transmission networks.