A substation is a crucial component of the electrical power system, functioning to transmit and distribute electrical energy. A transformer is a core device in a substation that modifies electrical voltage, and its failure can have a major impact on the system. A transformer's lifespan varies between 20 to 35 years and is influenced by factors such as load, oil quality, and fault frequency. It is essential to predict a transformer's lifespan to enable appropriate actions based on its condition. One of the methods that can be used for this prediction is the Health Index (HI), which combines historical and condition factor data with high accuracy. This study aims to analyze the condition and predict the lifespan of a power transformer at the Tello Substation, with a capacity of 60 MVA and voltage of 150/20 kV, using the Health Index method. The data used includes historical factors (operational time and loading) and condition factors (oil breakdown voltage, water content, Dissolved Gas Analysis/DGA, insulation resistance, winding resistance, turn ratio, and tan delta). This data was sourced from PT PLN (Persero) ULTG Maros and the Tello Substation. The results indicate that in terms of historical factors, the transformer has been in operation for 25 years and has reached a peak load of 93%, which is categorized as poor condition. However, in terms of condition factors, all test parameters are in very good condition and meet the standards. Based on the final calculation, the transformer's Health Index (HI) value is 3.22. This value indicates that the transformer is in good condition and is predicted to operate for another 11 to 15 years. The study recommends better monitoring and control of the load to prevent damage and extend the transformer's lifespan.