The importance of oil-immersed transformers in the electricity transmission and distribution systems are an obvious axiom in the modern day's power systems. Temperatures on core, oil, winding and structural parts in oil power transformers represent the most important limiting factors for their loading. This paper present an optimized thermal-electrical model to calculate the temperatures in the all active and passive parts of the studied transformer under unbalanced voltage condition. The important factor that influence the transformers performance, as well as thermal life, are load and ambient temperatures. So, estimating of this parameters during the real-life of transformers are useful for determining the loss of life (LOL) and remaining life of insulation. In this paper, Monte Carlo simulation is used to create the artificial history of load and ambient temperature based on average daily load and ambient temperature. This artificial history can be used to estimate the daily and seasonal loss of life of distribution under unbalanced conditions. The proposed optimized model is verified by the results obtained from the measurement carried out on 250 kVA, 34.5/0.4 kV, distribution transformer. The experimental results demonstrates that the proposed thermal model provides reliable results. (C) 2015 Elsevier Masson SAS. All rights reserved.